In teen fiction, your primary responsibility is to your teen protagonist. It’s her story, above all else, and your readers care about her struggle to overcome a situation, be it one at school, one at home, or one that involves a murderous, abusive parent. Your story must end with your protagonist finding a new maturity or understanding of herself and how she can live her life in full knowledge of her father’s crime(s). It’s not about a tidy ending, with Dad making good. Sometimes dads don’t make good. If being true to the story, the era, and the culture means that Dad shouldn’t get rehabilitated, then don’t force it for the sake of a pat ending. You want a satisfying ending, and that satisfaction will come from your protagonist’s emotional empowerment.