Neil, you're certainly right that people harm others in situations in which harming them is against their own interests.  There are loads of examples of this.  It's also true that the tendency for people to polarize into mutually antagonistic groups need not be selfishly motivated (or, indeed, motivated at all).  But polarization and antagonism need not involve dehumanization.
I don't have any problem with the proposal that we can treat others badly, cruelly, or even lethally without dehumanizing them.  Most cases of racism, for instance, are not cases of dehumanization (in my sense of the word).  I think that dehumanization is ONE (very important) way that we both motivating violence and disabling inhibitions against performing acts of violence, but it's not the only way.
David

If you know you will never need kindness from another you can feel free to be unkind to them. But how is this possible? How, that is, can we know kindness unneeded and how can we know, as a separate matter, kindness unneeded of this person or this kind of person? These are two issues complexly entailed to each other. One is intimate, can I know in any sense at all that I will ever not be in need kindness? And the other social, for how can I know I will never need kindness from this one person or this kind of person unless there is a social context supporting the unkindness that can result? For there is one thing for which we are always in need. Understanding. It is no trivial matter that we can speak and listen to each other so fluidly. The origin of such a mysterious ability is the certain knowledge that we do not understand and are not understood by each other. By the time we reach age four or five we have already reduced the need for kindness in this to such finely drawn terms that we take it for granted that the whole system of language is somehow mysteriously just given to us, or "wired in". But this reflexive prejudice does nothing to reduce the need of the kindness of others in responding to the misunderstandings we encounter. This kindness is intimate, for there can be no term in the taken-for-granted language that can resolve the issue. Only an intimacy amongst us that breaks through that impediment of established terms can so revise the language we share that that language grows more capable of supplying the facile meaning through which we are able to forget how much in need of kindness we are. Habermas wrote voluminously in an effort to bring that dynamic of need and intimation into the "public sphere," in a futile effort to render the language bureaucrats use more capable of that kindness we need to intimate meaning not yet available in terms of the broader society. But the main issue is to distinguish the personal dynamic from the social support required to produce the more limited meaning we can share in a social context in which it is possible to forget how much we need and how much kindness we need of each other.

Okay, I think I'm a little clearer on what I want to say after reading the above comments, and I'm less sure that I have a real disagreement with David. 
@Grue: I think we shouldn't talk about "the" default position. I think what happens is this: when we experience another entity of whatever sort, we consciously and unconsciously put categories on it (RED, INANIMATE, SHORT, SQUARE, etc.). Now some of those categorizations will be more salient (RED!), and others less so (SQUARE, for example).
My claim is that the us-them dimension ("He's BLACK!") of humans experiencing other humans is typically *more* salient then the human dimension, which sort of just sits in the background most of the time.
Still the human dimension of experiencing other humans may be, even if not always that salient, a sort of backstop the typically prevents us from being too cruel. Dehumanization may then be an effort to remove the backstop--a psychological technology after all--as David argues.
But still, I think the start of the cruelty in just about every case arises with the us-them dimension.  

I think an important factor is a limit to compassion.  It's said that our biology allows us to fit roughly 150 people into our sphere of empathy - we can actually share empathy with only that many people, and only after getting to know them.  With everybody else, we can use logic to know we should feel compassion towards them (I know they are human, so i should care for them), but i dont think true compassion is felt with such strangers and acquaintances.  This is what allows people to do awfult things to other people.  They can see them as people, but not empathize with them.

It seems to me there are several closely related but potentially distinct ways of regarding and experiencing another in play here:
1. Regarding another as a fellow human being.
2. Regarding another as a fellow person.
3. Regarding another as one of us.
4. Regarding another as one possessed of an intrinsic worth equal to one's own.
5. Regarding another as one to whom one has moral obligations.
6. Regarding another as one with whom one can or should empathize.
7. Empathizing with another.
There may be other relevant ways of regarding and experiencing others that should be added to this list. With the exception of 4, I think, all of those listed admit of degrees. It's interesting to consider how each of these modes of regard can or should relate to the others. We can ask which ones are or should be necessary and/or sufficient for others.
How we answer questions about the logical relations among these modes of regard may help us to get clearer about what we mean by them. But will it help us do a better job of honoring the dignity and worth others? Will it help us regard, experience, and treat others as we ought to regard, experience, and treat them?
Only if we are actually committed to working on ourselves, to becoming more ethical persons.

Considering a human or a group of humans as ?less than human? cannot morally justify treating someone badly.
Dehumanization does not explain why somebody would treat someone or something else in a wrongful way ? an SS Officer would consider there to be superior and inferior races, with Jews being not a race, non-human, and according to the concept of dehumanization, he would justify his lack of morality. And yet, this same officer would treat a dog (non-human, i.e dehumanized) well (with some sense of morality). So dehumanization cannot be the sole explanation for racist behavior.
We could think of it in this way: a person suffers a dehumanization of himself in a transitory moment, therefore becoming a ?non-human? with no moral compass, and thus treats selected others as belonging to other ?non-human,? perhaps animalistic, groups in an aggressive manner. Therefore, the topic becomes not dehumanization but animalization of the self.  

I agreed with Mr. Smith and I also experienced that some humans not treat to other sincerely even though they treat like animals and if they treat so they must be had interest behind this ,but humanity not teach this type of things . I already discuss this topic when I was UK Expert Assignment Done Writers platform I raised this point to all of us.

Certainly an ?empirically oriented? account! If it really is the case that our ?us? vs. ?them? drive is so central to our experience of other people, can we be held as morally accountable as we would like for acting on the various ?-isms? towards which this drive naturally inclines us? In fact, recent research on implicit bias adds force to this account. This is not to say that we should discontinue our pursuits of defeating this drive (where it leads to immoral action). Rather, knowing the centrality our biology plays in creating these ?us? vs. ?them? groups might help us combat their influences.