Well yeah, for Aaron to go from someone like Jackson to someone like Robert, there must be something more there than just sexual chemistry and it's been long enough to not just put it off as a rebound shag.
Aaron lost everything after Jackson though, so maybe anything that makes him feel that wanted again ticks a few boxes, whether they be a psycho or not
I'm not sure I agree with Robert trying to kill everybody. Good as the actor is, old Robert was a bad boy but he was never this much of a nutcase