Oh boy.
If this is story is planned to end with crazy plot twist i suggest you try to figure that out.
I'm thinking that the girl maybe early adult is living in an apartment with her (insert mental illness) twin and is put into several life threatening scenarios such as car crashes, drugged drinks etc. She doesn't know what is going on but she eventually, though means of spying or accidental, she learns that it's her twin. Maybe some twilight zone hitchhiker he keeps appearing anywhere sort of ordeal. I don't really know what genre you're going for but I think a very realistic interpretation and non-paranormal would be pretty awesome.
I don't have many ideas right now on murder motives besides some sort of schizophrenia or brainwash/temptation. But the story sounds great.
Best of luck!
