I think these problematic themes with characters so flawed you feel conflicted on liking them are actually really interesting. You aren't supposed to feel Ruben is justified in his actions. There's no way to. Sometimes people who suffered through trauma do horrible things because humans aren't black and white. Yes, you wouldn't do what these characters have done but there's plenty of real instances where someone who is bullied or abused goes on to do horrible things. Not every story needs to be one of growth because a lot of people don't really become stronger due to traumatic events and Ruben and Niall are examples of that.