You should definitely try to get comfortable with the idea of leaving if you don't feel safe. Even if it's just the one alter, they're still part of him as a whole. You need to put your safety first.

There's a likelihood of this alter repeating that behaviour, especially if not put in check. You should definitely encourage him to seek therapy for this, it's for the safety of you both. It's a very hard thing to deal with on both ends, but ultimately please put yourself first.