i say one of the reasons he's bad is becuase his mother (Petalcloud) traded him to her Sire (Rockwing) he felt abandoned, and he was afraid of Star, and that he wanted to take over the herds. SPOILER ALERT I was really mad when Frostfire Horsenapped Morningleaf and killed her protectors, and then he hid her in the lava tubes, (still spoiler alert) in the end he ended up working with Star to rescue his mate and the other trapped pegusi. SO hr turns out good in the end
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?