: Should my main character make a ginormous mistake? I'm rereading my draft, and there is a part in the book where when the main character is helping others escape prison, she accidentally reveals

I say let em' die.
My reasoning? Simple. Protagonists are human too. You can't expect them to be perfect and neither will the reader.
I don't know about the rest of you. But I read books for two reasons. The first reason is obvious: because they're entertaining. But the second reason is different: To show that 1. Even heroes make mistakes. And 2. To remind myself that nobody's perfect, even the protagonist of the book I'm reading. I've never really been the type of guy to read superhero books or comics, simply because the people in those types of things are too perfect. Let's take Superman, for example. If you could name 5 major mistakes that lead to death or something of that magnitude I'd probably pay you. A lot of the time heroes are shown to be perfect, but no one is. So I say that it's definitely the right choice to make your protagonist make that mistake. The reader isn't going to hate them, and if they do their hate will be short-lived. But either way, who really cares? It's your book and you should do what you want in it. If that's making the protagonist indirectly kill someone then make the protagonist indirectly kill someone. I support you completely and I'm positive the readers will too.
That's also why I hate your "backup plan." Once again that plan makes the protagonist look perfect, always cleaning up after other's messes, and never messing up themselves. This isn't Thor. Your protagonist (I assume) isn't a god from Asgard who has to be perfect in order to keep him big metal hammer. So please, don't make her seem that way.

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Yes. It’s fine to have someone die. Just make sure that you don’t only kill side Characters, because then it makes the main characters seem immortal, and the reader expects them to turn out okay.
The way I avoided this in my book is creating a main character who’s sole purpose is to get really important, draw a lot of empathy from the reader, and then be killed in a horrific tragic way that was the main characters fault.

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I have a slightly different take from other people on this. I don't think it is true protagonists should never be perfect; it might depend on what we mean by "perfect" but I think interesting and compelling stories can be written with characters that don't have deep flaws (or not flaws the author or reader sees as such or cares about). I think giving flaws to a character just for the sake of them having flaws is misguided. We talk about "humanizing" characters but that doesn't have to take only one shape. You can engage with a character's internality, show them having relatable doubts and fears and struggles and joys and internal contradictions without giving them a trait you feel is a "flaw" (for example here is someone arguing that good stories can be written about a character that is as perfect as they get). You can also have a character who you don't explore particularly well, but you know they "should have a flaw" so you make them clumsy in a way that has little impact on the story, or conversely have them run over someone in their car and not actually explore how awful this event is, but yay your character is flawed it's fine. I think we should not ignore the potential issues with a character being flawed in a way that is unacceptable to the reader, I have absolutely rejected stories because the protagonist was flawed in ways that did not make me want to read about them. Or more precisely I should say: their flaws were written in ways that did not make me want to read about them.
I think the important thing to consider is, why is a flaw a flaw? What makes a perfect protagonist perfect? The answer I think is the moral convictions of the author and reader. An author with well-thought out and consistent moral convictions will present a perfect protagonist in ways that highlight and defend these convictions. They will present a flawed or even villainous protagonist in ways that also highlight those convictions, by presenting a coherent view of why those flaws are bad, how they impact the protagonist and those around them, and so on. This will in turn make a story that is coherent and well-constructed in terms of themes, plot consequences, emotional resonance, what have you, and readers will respond to that.
I think the danger with writing "perfect protagonists" or the kind of flawed protagonist that makes you nope out of the book, is when the author does not have a coherent moral philosophy or has one that is so abhorrent to the reader that they cannot stand reading it. A reader can forgive a flawed protagonist; it is much harder to forgive a protagonist who acts terribly (or in ways the reader thinks are terrible) but who the author clearly thinks is perfect, where the consequences of the terrible actions are not explored in their terribleness and who faces no negative repercussions for their behavior. It is similarly hard to forgive a protagonist who is presented as flawed, but where the author seems to have little concept of how flawed they've really made their protagonist, or treats as flaws things the reader thinks are fine while treating as fine things the reader thinks are awful.
The issue IMO isn't just that we want to be morally validated by what we read, but that insofar as a plot has moral elements (and any story that worries about its protagonist being perfect or flawed has a moral element to it), like every other aspect of the work they need to have a realism or at least internal coherence to them to avoid pushing the reader out of the story. I see it as a form of suspension of disbelief. In a fantasy or science-fiction work you can accept some foundational premises and then enjoy the story insofar as everything derives from those premises, but you start getting in trouble when more and more inconsistent elements are thrown at you such that you spend more time making the effort to suspend your disbelief than enjoying the story, or find the story drained of tension because you cannot anticipate what will happen next given all the inconsistencies. Similarly you can temporarily adopt an alternate moral framework (within limits) to enjoy a story, but if it is badly thought-out then one has to constantly update the "moral suspension of disbelief" as actions get presented as good or bad with little in-universe consistency, and it becomes hard to root for an outcome if you aren't given a stable value system to judge outcomes by.
Here is an example of what I mean with how someone can enjoy a work of art they morally disagree with, because they think it presents its own moral universe well (and how it contrasts with one that doesn't) :

In your story, what do YOU think about the mistake you propose your character make? It doesn't strike me as a shocking mistake in the universe of mistakes action protagonists make, but I find it very interesting that you are worried about it. Do YOU think this is an egregious mistake? Why? Do you imagine the feelings of the friends and loved ones of the victims, and what they would think of your protagonist? Do you have a mental view of your protagonist as someone righteous, wise and reliable and you feel this action might be too far out of character for them? Do you feel you, as a moral arbiter, would not be able to respect this character as much as you want your reader to respect her? Conversely, do you actually think it's fine but are worried a reader would disagree? If so, why do you think it's fine, how would you argue your case to this hypothetical reader? I think this is all valuable to explore! And it should inform your writing. As you explore it hopefully your own views may become more coherent and something you feel confident in, and once that happens it matters less what your readers think - you can try to sell them on what you think. Maybe you'll decide that no, this is not a mistake you feel is forgivable for your character... If so I think it would be a bit cheap to have it just not happen, or happen to someone else. Because if it just happens to someone else for your own convenience, that means your character is just lucky to be "good". That's not very morally compelling! If this mistake is so awful that your protagonist must not make it, but someone easily could, what does it make the poor sap of a secondary character who did make it? Do they have a basic character flaw your protagonist lacks, is your protagonist conscienscious enough about human life that they will put the care into not making this mistake? Then portray that, think about what it would take to be such a good and competent person so as to not make that mistake and use your protagonist to show how this good person would be and work. Is this secondary character a good, competent person but the mistake is still a horrible one you find hard to reconcile with, if so how shall they reconcile with it? Is this moral journey interesting to you, and if so might it not be worth giving it to your protagonist? If you struggle to reconcile being a good person who has made this mistake - what does that mean for the world you built, that this is a thing that can happen? How do people exist in it? Can it be changed, should it be changed?

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Well, first off, I kind of hate your backup plan. It's just... simply because someone is the hero of the story, doesn't mean they have to be unflinchingly, superhumanly heroic at every turn. The larger-than-life hero archetype has been done to death. Sciborg is right to invoke the Mary Sue trope. It's not that it doesn't work, it's that it's boring. Your original idea is far more interesting.
As far as how the audience takes it, that'll all depend on you. You haven't given us enough information about the outcome of that event to interpret its effect on the story, and how your audience views the protagonist will hinge entirely on the outcome, not the event itself.

Is the person who was killed a stranger, a random scene extra with no personal connection to the protagonist? Does your hero shrug off the event and go about her business with barely an acknowledgement of her role in the other character's death? Then, sure, she will look like a complete asshole, and the audience will want nothing to do with her. Would you?

Is the hero affected by the death, deeply and profoundly? Does she spend the rest of the story haunted by the memory of the dead character? Well — maybe good, maybe bad. It's possible to blow that kind of thing by failing to sufficiently build a foundation for it before the incident. If you want to dump something like that on your story, it's gotta be able to support it. A strong emotional reaction like that requires a strong emotional connection between the characters, or it will feel hollow to the audience. OTOH, if the character who's killed is the protagonist's relative, or close friend, or the past associate she was attempting to rescue in the first place, then you've built a plausibly devastating event that you can use to shake her right down to her core. At which point you've got carte blanche to rebuild her in a dozen different ways, should the story require it.

If the protagonist doesn't have a connection to the character who's killed, can you proxy those strong emotions via a third character who does? Perhaps the dead character's brother, or child, or best friend, is also among the group being rescued, and has to deal with both the loss of their loved one, and the knowledge that your main character is partly to blame for that loss. Now you've created a believable, totally organic conflict engine that you can mine to create tension between that character and your protagonist. (...Holy crap, did that metaphor get away from me.)
If that's the case, then the audience reaction comes back to: How does the protagonist handle the events that follow? Does she take responsibility for her mistake? Does she respect and validate the feelings of the surviving loved one, even when they manifest as anger or disrespect directed at her? Does she make a promise that, while she can't bring back the lost loved one or ever undo the mistake she made, she will do everything in her power to succeed against their common foe / mutual oppressors, and ensure that $character's death was not in vain?

Your audience won't hate your protagonist for a mistake, or for her role in events that were beyond her control. But they will judge her for everything that happens after.

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You've really got two questions here:

Is killing too brutal for my book?
Do my characters need to make mistakes?

No one here can answer #1 for you. It depends on your audience and the tone of your book. I won't go so far to say that books for kids shouldn't have killing; people die in Harry Potter and even in nursery rhymes. You should be careful about how graphic the descriptions get if your audience is young though.
As for #2 , mistakes are often an important plot point, especially at the beginning of the story. Many stories start with the main character making a mistake that starts the central conflict. Mistakes are also important when it comes to making characters relatable. No one in real life is perfect, so a perfect main character often comes off as flat.

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Does it serve the story? Then, yes.
Putting that particular mistake in will alter who your target audience is, though. Some people will find it too brutal; some people will find the lack of such things means that the stakes are too low.
Also, how you treat it will affect how your audience reacts to your character. How serious the mistake is, whether it was neglience, what she did to try to retrieve it will all affect that.
You may need to rope in beta readers to help determine what the effect is.

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Yes, absolutely. Protagonists aren't perfect.
In many ways, protagonists making mistakes helps to humanize them. Of course, it seems wrong to write a scene where your hero, the good guy of the story, accidentally gets somebody killed or completely screws things up for other characters - after all, won't that make them unlikable? Doesn't everybody hate that fool of a Took?
I say no. (And of course we still love that fool of a Took.)
For one thing, every Hero's Journey needs a falling action, and the trope of the hero falling out with the other characters only to reconcile with them later is a well-known story device. Done right, this scene and its consequences will help to build the friendship and the relationship between your cast, as the protagonist has to find a way to make up for what they've done and ask for forgiveness from the other characters. They also have to struggle with their likely trauma from what they did, maybe even some PTSD, as of course survivor's guilt and the pain of their mistake will eat away at them and affect them in the chapters to come. This is all excellent for creating inner conflict for your protagonist, and making them much more relatable and three-dimensional. If anything, I think this storyline you've presented, done right, is a great idea.
We are all flawed people, after all, and we empathize more with characters who are flawed and make mistakes, not characters who are perfect in every way and always do the right thing in every situation. (See Mary Sue.) Nor will readers enjoy characters who always, constantly "swoop in" and save the day with perfect solutions after the other characters mess up, because that gets old after a while and makes the protagonist out to be some kind of savior character at best and an author scrambling to fix their plot at worst. (See Deus Ex Machina.)
So if your character makes a mistake that gets somebody killed, will the reader be shocked and horrified? Yes, probably. But will they hate or dislike the character? Not if you've done a good job of making the mistake realistic and understandable, a mistake anybody could make in their shoes, and made the character feel regret and remorse and work to fix what they've done. If anything, this scene could improve the story, and flesh out your protagonist in a great new direction. But of course, as with all things, you'll have to write it with care.

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