Why is he angry, what makes him treat you like that? This particular alter's likely acting out from a very injured place. As much as I hate being "sensitive", I was never evil. I had a purpose, still do. A job to fight off any threat to my system. For me, that job is the most important, before having a laugh with someone. I need to trust, I don't trust. It took something special and a lot of therapy. Our girlfriend never treated me badly, but wouldn't allow me to treat her any other way than respectfully, which in turn my respect became admiration for her.
My host is sweet, wouldn't hurt a fly and, that's why I had to do it. Using the term "evil" isn't going to cure this alter's anger which likely stems from something deep. If you want to help, you'll offer respect and a firm foot in the *** when he can't understand that you deserve the same respect. I doubt the alter will change right away, but give him time. You love your fiancee, so try to understand and appreciate each part of him, I guess is my advice. Who knows, the alter might turn as soft as jello, as I did.
Best of luck.
-Trev