Oddly perhaps, I have the same sense about this as you P-Marlowe. I'd say he's a pitiless narcissist who may actually get his jollies from the physical act of killing, and takes lascivious pleasure in performing "ceremonial" acts of abomination in the sight of God. It's his way of giving God "the back of his hand," so to speak. Not to mention his fellow human beings.
As to why such men have no fear of assassins, maybe in their crazed mental world they believe they are under Satan's protection. But if so, in this case, Satan was not a deal-keeper. Whatta surprise.
You said you thought such men are "demon-possessed." I certainly agree with you there.
p.s.: I'm keeping Dr. Tiller in the present tense, as he is still very much with us in spirit.