Board Thread:WARNING ALL USERS False Info\Spoilers/@comment-96.238.1.154-20130804085459/@comment-10123851-20130805072806

I'm guessing its because he killed a member of his own family, as far we know the alpha pack didn't do this, they just killed their own betas.

Taking a life seems to have a big impact on werewolfs, i.e. an innocent life gets a beta blue eyes, a member of your own pack gets an alpha more power.

So it just makes sense to me that killing family perverts your true form, the interesting thing is derek also spilled family blood, so if i'm right he has a beast in him too.