We wonder at the people doing dreadful things, how could this happen. We only ask 'how' of course, if we have a glimmer that the world is not, black and white. In fact, that may be the only generalization that holds most of the time: Reality cannot be divided into two things.
And, skipping the binary nature of rational thought itself, which we address frequently in other posts, let me suggest now, that this tendency to divide everything into two, is basic to the human ego (hence its prevalence) and specifically, can shed light on how, 'people can do dreadful things.' It may turn out that horribleness is just an extension of that which is ordinary, ordinary, because it defines us all.
The human ego must separate itself from the world when people grow up. Whether of not this division of self from world is a coherent stance, it in fact happens to us all, and so, must be healthy on various levels.
For example liberals often mouth off about how selfish rich people are, and seem to think the rich are the source of all evil, that a flattening of wealth will produce human progress. Any real progress must include questioning such simplistic analyses. We use it here as an example of this division of self and the rest, the world, and how such a division into two, is a sign something has been very confused in the analyzing. My example of the self and an alternate world of wealthy selfish people is meant to illustrate the inadequacy of binary approaches. But by way of a segue to our last example, let me mention that among other things, the rich provide a variety in the world and variety is critical to growth, although, of course, there are varieties of variety.
Another ordinary approach is that where the self is the judge of the evil of the rest of the world. This I hope I have explained already, is a basis of everyday psychology. The division into two; the self as defined as separate from the rest of the world; and that otherness, the rest of the world, as the source of the problems faced.
When we criticize people who do horrible things, we reveal our own ignorance, for these people are acting on the same psychological function as that which helps define us all. The evil is 'out there'; the evil (problems) could not be WITHIN ourselves: for such division is a necessary aspect of creating a human ego---- separating yourself out from the world. The people we criticize are puppets themselves and so how can someone with insight call "THEM" evil? Dividing the world into us and them is one of the oldest, and certainly one of the most illusory, motivations in human history. Motivations which may be necessary, but that does not mean everyone has to believe in the division. Not everyone has to believe in these motivations, just most.
This division certainly seems to be obvious in stuff happening now in the middle east. I am not saying we do not wage war against them. I am saying we do not assume they are different from us. For we all fall into, indeed are defined by, this mental proclivity for dividing the world into two. And if we assume the world falls into us versus them, then we are just imitating those people we dislike. For how 'they can do terrible things' is because all the problems in their world are "out there," and therefore personal, internal alteration or growth in thinking is not even called for. That, and yeah, they're pissed off.