now i’m really curious what drug they were taking that makes you kill your girlfriend
I was also thinking it was a bit too much. At least the manner in which she was killed. It was likely some sort of stimulant.
Stimulants can enhance emotions (happiness, anger, sadness, horny-ness), so I think it could have helped fleshing out the why a little bit more, e.g. did she get enraged (the drugs enhancing her anger) about her gf's behaviour and their living conditions? Not that it's a justification. Was she overly eager (the drugs putting her in a horny haze) in some type of BDSM breath play and simply lost control?
While I do believe the author intended it to be more open to interpretation it does cheapen the understanding of what drugs actually do to a person. I would have liked to get a better view on her emotional state before/ during the killing.
Though I did like the portrayal that the drug did have different effects on the both of them (at least in terms of addictiveness).