What about the blunt trauma to the head? That surely happened before the strangulation. Would that be from a fall (accidental or because he pushed her), or would it be him knocking her with something like a rock or a tool? (Colonel Mustard with a wrench in the KitchenThe entries in the notebook describe a version of events that were meant to blur the circumstances to make him look less guilty. The autopsy proved that she wasn’t in the water, wasn’t dying of hypothermia, nor showed any reason why she’d be begging for him to mercy kill him. He strangled her because he was an insecure, paranoid, jealous little shit.

So that would leave only 2 options:
- He's a psychopath
- He was under the influence of 4D STS (through a spirit attachment or a download or whatever other similar explanation)
What do you think?