A lot of people seam to think Riversun will be a villain like her parents, I don't think so. I think Riversun will turn out like Bramblestar, Tawnypelt, and Mothwing from the Warriors series by Erin Hunter. I think Riversun will live in her parents shadow, but she will prove her self a loyal to Star at some point in future Guardian Herd books.
Star grew up in a herd that he had friends in, and he felt somewhat safe in Sun Herd. Nightwing may haven't had a life like that, so maybe this caused him to become a destroyer?