By all accounts, a profoundly obsessed and murderous weirdo.
It's always difficult to determine what goes through the minds of these types of individuals, and indeed what drives them to latch onto someone to such a degree.
We've all been a little obsessed at least once in our lives, but we've always had the capability of restraint and reason.
Others don't, and they're psychopathy acts as some sort of positive reinforcement that what they're thinking and doing is perfectly fine.
I imagine that wanting to kill, for lack of a better term, the 'apple of your eye' is a last resort for power over them.