As history bears out, there are times when these tendencies emerge, and we become murderous and destructive toward others whom we decide are bad. The evil, which children can own through role-playing, becomes projected onto a group of others, and crystallizes as a static identity. We have a remarkable ability to de-humanize these others so that even murder can be rationalized -- they are sub-humans, cockroaches, non-entities, and they need to be stamped out for society to be healthy.
This is an interesting point that I've thought about a lot, though I think from probably a slightly different perspective. Yes, people in power or sociopaths have dehumanized their victims as history has shown, but the rest of us in our retaliatory human nature have turned about and de-humanized them because of their crimes. Pre-empting Godwin's law here, it could be argued that many Nazis were just following orders or didn't comprehend the ramifications of their actions, yet the entire group is demonized along with the leaders. I don't want to go on much with this example because the morality behind "support the war", "support the troops but not the war (they're just following orders)", and "don't support the troops (they should know better than to carry out corrupt orders)" is grade-A flame bait on an internet forum.
The example that bears out more for me is the Columbine shooting because it was literally close to home when I was in high school. The killers were bullied and generally poorly treated by a small segment of their peers and generally ignored by the rest. That can give a strong impression that everyone is against you. They felt de-humanized and acted out against that violently and catastrophically. In their own rampage their victims were in turn de-humanized, but the saddest part of the story (for me) is how they were treated in the wake of the shootings. Media outlets who interviewed students and parents almost universally called them "monsters" and I heard the words "how could anyone do something like this?" over and over, as though we had to separate our species from whatever they were. It was like everyone said "WE would never do that (OK that's probably true that most wouldn't shoot people at random in a high school, but I personally believe the seeds of such actions exist in everyone, whether they take root or not) and we don't understand the psychology of someone so depraved and MONSTROUS, they're just so different that we don't feel any need to try and understand it, so we just label it "senseless killing" and move on. Trying to understand would be sympathizing with the madness of killers." The regrettable truth is that people treating other people as inferior, inhuman, and expendable is so common, yet I don't think we understand WHY it happens. Kids being bullied in high school is "just part of growing up" and kids shooting other kids without remorse is "inconceivable and inhuman" according to our culture and society, but I would pose that both leaves are growing on the same tree so to speak.
EDIT: On a separate but related note, I advocate using games as an outlet for these tendencies. I usually play the bad guy on the first run through. I find the villains are interesting characters. I commented on the "why are gamers jerks" article that, while I wouldn't want to see the Joker win, I have an innate curiosity to see what scheme he cooks up.