It's just as likely as not that this is the first time he's done something like this. There's always a first victim. Kevin Ray Underwood, who murdered, assaulted and intended to cannibalize Jamie Rose Bolin (a child) was a seemingly fairly normal guy with social anxiety and depression...until one day, he crossed the line and was no longer a "normal" guy. He got obsessed with cannibalism, decided he should eat someone, lured a kid into his apartment, and murdered her.
I was friends (not close friends, but certainly friendly) with him in a game chat. Talked to him for a couple of years. He was an absolutely average chatter who answered questions and sent out helpful items to newbies. His police interview is as honest, raw, and horrifying a portrait of someone losing their mind as anything ever has been. It's not light reading, to say the least...I don't suggest doing so unless you KNOW you can handle it. I can handle most anything, but I deeply regret reading it. However, between that and the articles about him, it's a clear portrait of how someone seemed to be doing okay until one day...they just weren't anymore.
Some people have obvious issues their entire lives, but some people's are waaaaay down deep and don't emerge for a long time.