Only he knows the why, but from a psychological point of view it seems to stem from just being a psychopath. His upbringing appeared to be standard, but the guy was just messed up in the head from the outset and took a chance opportunity to fiddle, got away with it and then that escalated. People say his charity work was his way of balancing the scales, that because he was doing good, that his sins would be forgiven, the series has a strong emphasis on that alone, but I don't think this is the case at all. He did the charity work because it gave him quick and easy access to those unable to defend themselves, and if they spoke up then nobody would listen to them anyway which is exactly what happened.
Some people are just born with neurons that fire the wrong way in their brain even though they are raised in normal households like everyone else, they just end up messed up and then do things like this, or shoot up a school, or shoot their family, or join a cult, or any number of other terrible things that people do when they are damaged it seems.
