Brackentail bullied Star through out the first year of the black foal's life, he even tried to murder Star. Most of us would have to agree it was pretty despicable of Brackentail to do that, but if you think about it, what would you do if you looked at Starfire from Brackentail's prospective?
Imagine this: someone you really cared about, was insistently hanging around someone you were convinced was a bad influences. What would you do? Sit by and let the person you thought was a bad influence be around the person you cared about, and potentially end up hurt? Or would you stop at nothing to lead your friend away for the bad influence?
I think that a lot of times we don't try to understand why the villain is the villain. We don't try to figure out how that character became evil. Brackentail ended up becoming a trusted friend of Star, but only after Star had convinced Brackentail that he wasn't a bad influence on Morningleaf.