1. What was the crime she committed with those other guys? 2. Why did she pick a nice guy to kill? Wouldn't it make more sense (well, to her, you know what I mean) to kill a jerk that looked liked her dad? 3. Where did she really get the scratches on her face from?
1. She was doing the same thing she was doing with Christopher, taking over their lives... and when no one loved her enough, she would lead them down a self destructive path, drugs/alcohol, and then convince them to kill themselves. "want to know how you can prove you love me?"
2. That was just how messed up she was... a perfectly nice boy, who would have done (and did) anything for her, isn't enough for her. it's never enough and it drives her insane.
3. She was telling the truth the first time, she's a cutter. Hence the razor blade that she keeps in the black box... and why she cuts his thumb, and convinces him to cut himself in the end. The song she sings is all about "Life Blood" or whatever, i think she was obsessed with it.