Emily killed herself because she couldn't deal with her masculine side because she is a closeted lesbian. When her masculine side actually speaks, he talks about how he wanted to kill her his whole life. It's because of the stigma associated with being gay. Even her parents sent her to shock treatment to fry the gay away. As we all know, those ex-gay camps never work and she came back damaged beyond belief, but brainwashed into keeping the man at bay. When the man is released, he comes out with a vengeance.