I can see why everyone is thinking like that but put your shoes in her place for a bit.
It's absolutely fucked why a murderer is saved like this in a cliche manner but, there are probs reasons why she is doing it (killing) and how she can't escape it so she probs started enjoying doing it because for her it is natural, she grew up that way.
Well if she joins mc's side, she'll have tons of room for character development so I don't see this cliche development as a negative.
Also, being nice to an enemy? Bro that nibba is from Earth not long ago, he grew up with different morals from different norms. Usato would never just kill someone because not only was this stressed out by Rose (don't kill people), he was also just a highschool boy who have never been in this situation (killing an enemy) before up until this point.