It isn't, it's a perversion, and the worst perversion mankind is capable of - the use of overwhelming power against the weakest and most vulnerable in any society, and a cynical abuse of their victims' childlike trust. Now, pick the bones outta that.
Is a serial killer a pervert? No. They are certainly mentally screwed up and, to protect society, should be locked up. Perversion implies choice and the severely mentally ill can't control themselves, ergo, no choice. Since such illnesses are often in degrees, not just "On or Off", we can't just lock up all paranoid schizophrenics and people who are attracted to children. If they break the law, then the law should do what is necessary to protect the public.
...and I was trying to give you a broader perspective on the problem. Let's be clear here; I don't condone anyone doing anything that harms another person. That said, if I come upon a rabid dog, I don't hate the dog nor blame the dog. The dog is sick, I pity it. Then I put it down because there is nothing to be done. No cure. No treatment. Nothing to do but give it mercy.