I'm guessing the murder mystery will be one of those 'I didn't actually do anything, but feel guilty because I couldn't stop bad shit from happening, so is it okay for me to be this happy?' variety.
Exactly what I was thinking. Her father might have died while doing something she asked him to do, and she blames herself. Or some freak accident happened that nobody could have predicted. Whatever it is, the fact that she feels so terrible about it and it's affected her entire personality seems enough for me to believe she didn't straight up murder him.