Brackentail and Frostfire were my favorites. I don't actually think of them as villains. They were both misunderstood. I also like Rockwing and Nightwing, they were both evil but I love evil characters because they are so interesting. The one villain I hate is Petalcloud. She is just heartless.
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?