: How to build suspense or so to establish and justify xenophobia of characters in the eyes of the reader? I am asking this as a general thing, be it a race that is never seen but heard about

A phobia means unreasonable, illogical, and excessive fear.

It might have gotten diluted in recent years due to politics, where it is often used against opponents who have the slightest dislike against a group or just merely fail to openly support them.

Xenophobia means an excessive, unjustified fear of foreigners.
If an enemy army is really about to conquer you, it's not xenophobia to fear them.
If they really did commit atrocities against you in the past, it's not xenophobia to not trust them. Or maybe it is, depending on the circumstances, but it has at least some justification.

You can make both those unseen foreigners and the "xenophobic" locals sympathetic, by making those past conflicts be based on a misunderstanding, or at least having been forced by unknown circumstances. The locals really fear them, because they really believe those atrocities happened. Maybe they didn't happen, or they happened but were due to some misunderstanding instead of malice.

There are many cases of literature and other works depicting well-intentioned extremists, who occasionally might happen to be right. Care must be taken to leave some dilemma in the work, to not be seen as trying to drop a moral anvil on the head of the reader.

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This isn't inherently 'hard', but you'll have some people use their higher brain to fight back against this no matter what. You use the same techniques propagandists use. You just point at something that happened that was bad and was done by the 'other'. Boom, you've got the majority of humanity on board there. You just need to make sure you describe the 'other' differently also. Even if they are other humans then you need to generally describe them in a more animal, evil, or different way.

The minority of humanity is the group that realizes that just because 1 person does wrong doesn't mean the whole bunch is necessarily rotten in terms of a race of people. To get them on board you need systemic underhanded racism, have members of the other race refuse service, talk down, in general be petty when dealing with the protagonists. Things a person knows to be racist but they might have experienced themselves, though the reason was not necessarily racism. The villains that are hated most are those that in general are often petty and relatable to real world scum bags.

If your protagonists raised in an environment like that and you show the reader this while people may not agree with the protagonist, they will understand his point of view.

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Xenophobia can efficiently arise from an Us vs Them mentality. To allow a reader to sympathize with a character who holds such feelings, you can present the situation as "They are against Us". Some ways would be to give the appearance of:

They help each other out a lot (but won't appear to help the majority group)
They keep to themselves. Maybe they feel safer among themselves as a result of persecution, but then when someone is bold enough to deal outside of their circle, it is for trouble.
They won't marry outside of their group. This can anger a majority group character more if the outsider group would date a majority group individual with no intention of marriage
They may actually hold some power: political, business...thus making some feel threatened by them
Give the majority group troubles of their own. If they feel they are in a precarious position, an outside group can be perceived as a usurper. Play on these anxieties hard but implicitly
They don't hold sacred what the majority group does. So when there are tensions a few of the outsiders might target those icons the majority holds dear, and in their anger categorize the majority in broad, sweeping, ugly ways

In short, apply filters to the perception of a group to make it seem like the other group has posed themselves as the enemy. "They started it". So when a theft occurs and they are enraged, it is not about the theft, but a litany of offenses that members of the outside group had committed against the members of the majority.
Sadly, history is full of examples of these cases, and worse, not just history

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Example: Steve Fields

The best example of such a character that I can think of is Steve Fields (played by Michael Harney) in Deadwood. Steve is a town drunk with anger issues and ingrained racist behavior (which he spouts off plenty of times). He doesn't mean to be evil but he's practically uncontrollable and incredibly volatile.

He's painted as a chaos element who the main characters do not want to interact with and pretty much avoid at all costs (other than when he's paying for drinks). He is loud, brash, unnecessarily combative, and a general nuisance. And yet I sympathized with him at the end of his story arc. How did I get to the point of sympathizing with a loudmouth racist and xenophobe? Because I could see it from his side.

There is a stable in town which is owned by two black people. At one point, one of their horses runs wild and runs over (and kills) a child. Steve witnessed it, tried to stop it from happening and tried to tend to the child afterwards.
The stable owners, knowing full well that their skin color is not working in their favor and thinking it's likely they will get lynched, leave town immediately.

Steve has a bit of a soft heart and takes pity on the horses that are now left behind. He takes up residence in the stables and tends to the horses. This also gives him a purpose and while he's still a difficult man, his life and character dramatically improves.
Many episodes later, the stable owners return and try to kick Steve out, since it's their property. This is where Steve the racist xenophobe flies off the handle and we encounter a situation where it's hard to pick a side on who's wrong and who's right.

Steve is a massive racist but he did tend to the child and the horses without expecting anything in return. The stable owners are unfairly subjected to racism, but on the other hand they did avoid all responsibility in the death of a child that they pretty much singlehandedly caused.

The problem is Steve's character (and educational level), and how he expresses himself. He screams at the top of his lungs and gets in the face of and threatens anyone who thinks he's wrong. He doesn't argue the point of fairness, or his good intentions. He doesn't argue that he's de facto taken care of the stable for a long period after it was abandoned. Steve relies on what he knows: the alleged inferiority of black people. And his argument is almost purely one of prejudice and racial bias.

But the viewer knows that Steve is a simpleton who's struggling to express himself, rather than someone who's actively trying to make a racial argument for their own benefit. Over the course of several seasons/episodes, we have seen Steve and how his lack of manners works both in his benefit and to his detriment. This clues the viewer in that Steve isn't pushing an agenda, but rather that this is who Steve is.

Above all else, the viewer understands that Steve's argument is not just based on a blind belief, but rather a misinterpretation of the things that do actually happen to him. Steve has been wronged. The people who are wronging him happen to be black. Those are two objectively correct facts, but Steve's mistake is thinking that the two are somehow related.

In general

So how can you create a xenophobic situation where the xenophobe is understood as non-evil?

In essence, this is no different from an alleged evil character who turns out to be misunderstood (this is a very common trope for plot villains and monsters), or a good character who gradually becomes the villain (e.g. Harvey Dent => Two-Face).

You have to approach the conflict (at least one of the perspectives) from an angle that completely omits the xenophobic element. Show us the person, not their political ideology.

Don't start from their xenophobia and work your way out, because the reader will think of the character as "the xenophobe" and won't bother much with understanding the finer points of their character.
Instead, show us their character, and only reveal the xenophobic element when the reader is capable of understanding why the character does what they do.

Show me a parent who is trying to feed their children, but struggles to find work to make the money they need.
Then show me that the work he is capable of is commonly taken by immigrants who are cheaper. Be subtle about it, or at least don't let the character get caught up on their ethnicity for now (even if the rest of the world does)
Now let this parent choose to get involved with a xenophobic (e.g. political or criminal) association that works to remove immigrants.

By taking this route, you've already laid the groundwork for the human element (feeding your children) to justify the character's choice (getting rid of immigrants).

Readers, just like all people (including xenophobes), are prone to prejudice. And first impressions matter. When a character is immediately presented as a xenophobe, readers will succumb to their prejudices just like the xenophobe succumbs to his prejudices.

Counteract that by showing us that they are humans with human emotions, an understandable life situation, and a justifiable need to look out for themselves. And then let those elements manifest themselves as (reasonably tempered) xenophobia.

From that point on, you can shape the events in that character's life that (coincidentally or not) reinforce their xenophobic ideals.

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Xenophobia / racism / prejudice / discrimination + stereotypes

It's all connected to an 'us vs them' mentality, which is naturally rooted in any species that survive in packs. Otherwise, it would be 'me vs them'.

In 'modern' societies, we should have the technology and information available to pretty much end the xenophobic tendencies - but it's difficult to go against our nature.

A basic buildup and/or recipe for creating Xenophobia etc. against "x group" of individuals:

Be introduced to the 'fact', that there are cultural differences between 'your group' and 'x group'.
Have unfortunate experiences with X group, either first-hand or second-hand, through relatives or people you know.
Continually get exposed to similar stories as the experiences mentioned above - Either from relatives/people they know or in the media - THIS HAPPENS A LOT.
At some point, forget that the stories represent individual experiences, and start to believe that such experiences are representative for an entire group of people, thus stripping them of their individualism as people.
Support, repeat, build on those stories to further develop the idea, that "they" can not be united with "us"...

Now, the cultural differences are where you can use your creativity to establish an authentic depiction of the society you're working with.

Cultural differences can be anything. Really. Some examples:

Believing in 'x god' or not believing a god, believing in souls, nature, life, etc.
Rituals, either based on beliefs, traditions, agreements, history, etc.
Supporting different political directions, ways of running countries, cities, society, different rights for minorities, etc.
Enjoying certain forms of entertaining: Bull-fights, Nascar, hunting, swimming, running, other sports, comedy (about what?), dancing (different kinds: attitudes, clothes, etc.), music, you can mention anything here - an individual can be entertained by as many different things as there are things in the world.
Eating habits...
Clothing
Language; either speaking a different one, speaking multiple, having an accent/dialect, etc.

A small exercise that could spark some ideas for working with these differences:

Think of the 5 people you know that have the MOST similar culture to your own, based on the criteria above (and anything you find relevant to your story)
After that, think of 5 people that are the LEAST similar to you, again, based on the criteria above.

Imagine, then, how easily someone could have an unfortunate experience, either with the people similar to you, or the people different from you, if they encounter them in a specific situation, where the differences are obvious.

Getting back to your question.

Any person, who has had bad experiences and/or a certain exposure to negative stories about X group of people, should be forgiven, to a certain extent, for believing that such are representative for other people that could 'belong to' X group.

We, the readers, would understand a character's xenophobia/racism/prejudice/discrimination, if we then get the slightest insight into what has led to it.

In your case of them flying off the handle and killing, you can tell, in your story, as it is in the real world, that SOME act this way, but probably not all.

Maybe your main character know that (some) people, that could be described as belonging to X group, are killing foreigners - but he/she has never experienced it first-hand, plus, the people that she knows, from X group, perhaps distance themselves from that behavior.

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If you're referring to how to make a sympathetic character be understandably xenophobic, there's several ways to do it, but the common unifying trait is establishing the alienness of the culture they're xenophobic against and why it is the character doesn't understand them/takes issue with them.

Interestingly, I'm in the middle of writing a novel which covers xenophobia/racism (in this case, against orcs), and it comes from relatively few factors in the protagonist's life:

1: She was exclusively raised in a mage cult that values magical power over everything else.

2: Orcs cannot usually perform magic, and by the cult's value system are on par with animals.

3: She's relatively unexposed to ordinary urban life outside of infiltration, so she hasn't seen orcs sitting around and chatting like ordinary people.

4: She's been on many missions in the wild, where nomadic, raiding tribes of orcs are common. These orcs often do not speak the common tongue.

As such, the information the reader has on orcs is that they're a non-magical, warlike, savage, incomprehensible race that cannot be reasoned with. As such, the protag's xenophobia is perfectly understandable... until she ends up taken prisoner by a relatively gentle orcish tribe run by one of the rare few orcish mages in history (who can speak directly to people's minds, skipping the language barrier). This forces her to accept orcs as people, albeit with a lot of mental resistance along the way.

In addition, I made her expression of xenophobia/racism semi-sympathetic (but still requiring change) by having her react to a fellow cult member behaving sadistically to an eager orcish teenager under his mercy. He, as a necromancer, is taunting him in the common tongue as he dies, getting off on his slow acquisition of control over his body, telling him he's going to raise him and use him to murder his parents, et cetera.

Despite viewing orcs as savage animals, the protagonist's throat fills with bile and she demands that he 'put him down'; that is, kill the beast to stop it suffering. She reiterates that the orc doesn't understand common, and there's no point torturing him. This shows that though xenophobic, she's not doing it out of a place of hatred or sadism.

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