the turning point *spoilers*
When I first watched this film, I thought that Johan decided to kill Lisa when she refused to get out of his car, but now, having watched it for the second time, I think that he got that impulse when she didn't calm down after he'd showed her his 'trophies'. He wanted her to know the truth but was annoyed when she thought him a monster, and she was too close (physically) and temptation was too much.
What do you think?
Hide the rum!