not surprise he killed her, from the start the hints of him being addicted to killing himself is there or something. fundamentally he still like her and wanted to save her but it's mix with his obsession, his inner monologue is mostly his consciousness to save her but subconsciousness started wanted to do his obsession.
when she give him that lip balm he said "this is starting to feel like a duty" I might be reading a bit too much about it but it seems like he is complaining to a chore that is a normal thing to do to some people like giving a goodbye kiss before living the house.
I bit that if he have to kill her again he would justify it to her.