I'm right with you. It makes no sense. I saw nothing that would indicate a troubled soul.
However, I don't believe it was drugs, steroids, mental health issues, neurological issues, etc., because of one thing- how he acted and tried to manipulate the cops, when they came to question him and/or get DNA swabs at his house on two occasions and when he was being questioned at the police station.
He turned his emotions on and off and acted "crazY' or seriously disturbed when LE was in the room, but the moment they were gone, he was wiping his nose, moving around, stretching, yawning...
That's manipulation and shows a lack of empathy. I don't know how there weren't various apparent signs of what this guy was before now, if, as it appears, there weren't, but if he hadn't been caught I have no doubt this would've been the first of a line of horrible murders. (If it was the first).