It is not wrong for the LW to want to leave her boyfriend because she is tired of his abusive behavior. Even though she loves him and cares about him, she does not have the power to make him better. Depression is a physical as well as a mental illness, and only medical attention will help him. I know this firsthand with my mentally ill husband. No matter how patiently I explained that his delusions were not real, he refused to believe me.
Like other correspondents have suggested, make a careful plan to leave so that you are not in danger. And if he threatens suicide, seek medical help for him.