There's no way to tell why he left her.
It is possible he made this decision on his own. And, contrary to OP, the whole world would see him as beta if he "forgave" her and stayed with her. If anybody should not have a scarcity mindset, it is someone like Pattinson.
However, this Hollywood we're talking about here. It is just as like that his publicist or some other actor(s) got in contact with him and said, "You want to keep yourself marketable and maintain a career? You gotta dump this b!tch. You stay with her and all your street cred is out the window. You'll not get too many more offers for parts because you now look like a wuss to everyone, not just the men out there."
Finally, I can imagine how many posts would be written about his beta-tude if he stayed with her. He may not be an alpha-dog, but leaving her was his only choice really, especially since her affair was made public.