Lol, and what makes you say they aren't realistic? I mean, there are people in history that have acted precisely as Opal and Voldemort acted. There are Damon Kronskis and Extinctionist-like people out there. People have watched other people getting tortured for entertainment, or fight battles to the death, that was what the Hunger Games I think was based around the idea of. (Wasn't it?) There were the people of the capital who were naive and didn't really understand the suffering that was being inflicted on others, yeah, but there were the people like Snow who knew what it was, and chose to go through with it anyway.
So as far as their actions go, I would say those characters are very realistic. Maybe not common, but then, neither are heroic figures who would risk themselves for others, like Harry, and Holly. You can believe that deep down a real person's thoughts might have been different, but according to the evidence we can see (their actions, their writings, etc.), these are exactly the kind of people they were, people who cared nothing about others, and only themselves. They were not born the way they were, rather they made a series of choices throughout there lives that turned them into basically little more than monsters.
Hm, well, on good and evil, then I guess that's where we're different. (; I definitely believe in an absolute good, and an absolute evil. There are shades of gray surrounding motivations for what people do, yes, (a man who murders someone else to protect his family or himself, for instance, or even if he kills an innocent person out of fear for his own life in some twisted scenario, I can sympathize with, but a serial killer, or someone who murders someone else to get money or whatever, who takes away someone from their loved ones with no remorse... well, I don't see any excuse for that).
I believe we are all born with naturally selfish desires and naturally good and kind impulses, yes. And I do believe that some people come from really messed up circumstances which have warped them, and many of the people who are messed up, any number of us might be just the same way if we were forced to endure those same horrible circumstances. But just because a good many of the people who do terrible things may have understandable reasons for having something wrong with them by no means means there are not people who just choose to become evil for really no other reason than that they just choose it. From what I understand, Ted Bundy came from a good home, and I remember there were a couple of young teenagers at a school who massacred a bunch of students (the media jumped the gun on this and claimed at first that it was because they were bullied, but it became clear later that this was not the case)
Psychological problems can and do play a role. But there comes a point where you can't just apply blame to circumstances (Now that I think about it, this was kind of a theme in AF (: *happy to tie it in* There was a scene in one of the books where Artemis thinks to himself something roughly like, "My future lays before me, and I can choose where to take it. I can't blame circumstances for what I become; I am my own person, aware of myself." Something like that.) At some point we just have to realize some people have chosen to make choices that are evil, for reasons that none of us can grasp. I think this mentality that no one can be truly evil has been both a good thing and a bad thing; on the one hand, I think it's made our society want to try to be more understanding of other people and where they're coming from, because Hitlers are probably rarer than Puritan-ish societies of the past have believed. But at the same time, psychologists and judges always seem to be wanting to give murderers and people with violent records second chances, when they have proved themselves such a danger to society.
Oh, and I'm definitely not saying the people fighting Hitler should have done to him what he did to other people. Fighting evil with evil... well, then that leaves no good or justice in the world. You don't fight evil with hate or rage or vindictiveness because, yes, that makes you the same as what you're fighting, even if you find ways to rationalize how you are better.
edit, *looking back on long post* whoa, I got a little emotional there. (; Too many late night dateline episodes, lol... But I did used to really want to buy into this idea that there's no such thing as evil, and everyone can get along if we just try. But some people... there's just nothing you can do. (Hitler I think was one of these. The world basically tried to turn a blind eye to what he was doing at first, because they really just wanted everyone to try to get along and they didn't think he would take his plans very far. But Hitler really didn't want to get along; he wanted to have the position of supreme power over the world and to kill everyone he didn't like...)
“After all, absolutely no one can help but suspect a criminal, liar, and manipulator of committing crimes, lying, and manipulating. And of course, no one is more aware of that simple fact than Artemis Fowl.”
Opal sets into motion her most diabolical scheme yet, to frame Artemis and turn his closest friends against him. Only this time she has a new calculating partner who knows Artemis better than he knows himself. [An Artemis Fowl fanfiction, set after The Atlantic Complex.] https://www.fanfiction.net/s/8336552/1/Noble-Heart
...Shameless self-advertising, guys! C;
(And if you're really bored: http://axxonu.deviantart.com/gallery/28912232/Artemis-Fowl
AF fanart. ;J)