@gspgreases said:

What the heck was he thinking? Let me kill my girls father, she would so be with me after that. Shes gonna hate you bro.

Yeah. The movie really did a poor job with this subplot. There was literally no reason established why she would ever be okay with the boyfriend murdering the dad, at all. And while it's possible that there was another (cut) scene that might have fleshed this out a bit more, that doesn't seem likely to be enough either. Like, unless the dad was an actual abuser of some kind, there is next to no way the boyfriend's plan would work out. But then again, there's not a lot of people acting particularly rationally in this one, so ..