The progression of criminality is very common, as is using a fantasy demonic other to escape responsibility. Gaps between crimes is not that uncommon among the organized types which implies a degree of psychological control.
Low eroticism ... :) I was going to suggest he was a really boring serial killer... :), no hints of perversion or necrophilia (the most hideous of sk behaviors, but nevertheless common). He was just demented and angry about his sister's sick victimization.
Check out this guy:
https://www.youtube.com/watch?v=HjHS_oZf3ak