I went through a period where I only read about serial killers, must have read around 30-40 books. People couldn't understand why I'd do that, they thought I was weird, and more than once hinted I might be an aspiring serial killer.
But honestly, it was entirely innocent. It all started with me reading East of Eden by John Steinbeck, and I came across this passage: "Maybe we all have in us a secret pond where evil and ugly things germinate and grow strong. But this culture is fenced, and the swimming brood climbs up only to fall back. Might it not be that in the dark pools of some men the evil grows strong enough to wriggle over the fence and swim free? Would not such a man be our monster, and are we not related to him in our hidden water? It would be absurd if we did not understand both angels and devils, since we invented them."
It got me thinking - We're quick to praise our angels, we take pride in them, many seek to understand them and take inspiration from them, so perhaps one can better one self as a person. But when it comes to our demons, we try to separate them from us; "He's not human, he's a beast!" we say. To truly understand our angels, we got to study our demons, because how can one understand goodness, if one doesn't recognize the evilness within us? We should seek to understand our demons, so we do not end up walking down the same path that they did.
So i started to read a lot about serial killers, to learn what I could about these people. And quickly I realized that these demon have a lot in common with you and i - Most are not hateful mudermachines that strikes at anything that moves. Indeed, we have so much in common with them, that many of them are perfectly capable of blending into our society.
There's no single reason why these demons walk the path of evil, but it's safe to say it's not entirely their own fault for ending up on that path. Often, they have had evil done to themselves from family, friends and society in general. Some struggle to find their place within society, and often end up feeling rejected.
Many times there's a certain rationale behind the way that they act. Their train of thought isn't harder to understand than, say, many conspiracy theorists. They strike out at those who have, or represent those who have, wronged them. A classic example is Ted Bundy going after young women from the upper middle class - All his life, Ted Bundy wanted to be a part of the upper middle class. But no matter what he did, he could never achieve that status, but he certainly felt he deserved it. So he ended up feeling rejected - And therefor sought to punish this group of people, by taking away their most valuable thing - young women. Another one is Mark Essex (who was a spree killer though, not a serial killer) - He was an African-American who grew up in a small town where there wasn't a lot of racism. After joining the navy in 1969, he experienced racism on a grand scale. This eventually pushed him over the edge, and he took out his anger on Caucasians (primarily police officers) in L.A.
After having read all those books, I felt I had learned a lot about what it means to be human. The most important, but obvious, lesson I walked away with was: Both our angels and demons have capacity for good and evil within them.
... Well, I honestly didn't intend to ramble on for so long. But hey, the topic at least had serial killer in it, so it wasn't entirely off-topic!