I absolutely love the character Brackentail! He has such a big change of heart in the second book! He becomes so devoted to Star and Morningleaf. I also do think that him and Morningleaf should end up together.
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?