The answer to your last post is - you can't. You can't change him - you can ask him to and threaten him but ultimately you'll only things back to how they were at the start if he is prepared to work with you to achieve that.
I know you and this guy were friends but what he's been doing is abusive.
Ordinarily I don't think there's anything wrong with self-harm - its often just a coping mechanism and quite a common one - some people pick up a chocolate bar - others pick up a razor. in this way its ok and shouldn't be looked down upon because any coping mechanism is better than none.
However, this person you're referring to doesn't seem to be using self-harm as a coping mechanism - he does seem to be using it as a tool aimed at emotional blackmail. In addition calling you a slag isn't ok.
Personally, since it's you posting and not him - maybe the question should be - why are you staying around someone who's treating you like this? You sound like a really caring and kind person - but that doesn't make you responsible for other people at the expense of yourself, and taking care of yourself by distancing yourself from someone who consistantly hurts you does not make you any less caring or kind.
This bloke sounds fixated on you and I would guess you aren't therefore the person to help him - you may feel like the most important thing in the world to your friend. He may very well think you are - but I suspect you are less important than this game he is playing.
Either way the answer is (in theory) simple...
Set very firm boundaries with him. Make them so fixed there's no room for discussion - write them down and go through them with him if you have to
some of them would be -
1) He will not cut himself in front of you
2) He will not tell you about him cutting himself - that it's ok for him to cut himself (if you think it is) but that he's not allowed to discuss it with you AT ALL (he may then stop if he's doing it for emotional blackmail reasons)
3) He will not call you anything (you'd have to agree not to call him anything also
You would need to tailor the others to suit your situation but make these boundaries rigid and tell him and mean it that if he breaks them you will end the friendship.
That way - if you are more important to him than the game you'll be able to build on the friendship. If you're not more important than his game he'll break the boundaries and you'll know where you stand - hopefully that will be what you need to realise that he's not good for you and you will end the relationship.
More than anything I want for you to protect yourself from being abused by this person. Just because he is acting out does NOT mean he is more important than you - you need to protect yourself as much and MORE than you support other people. Otherwise you're not going to be as helpful a support to people you could genuinely help.
Hope whatever you do works out for the best xxxx
(oh my god how long is this post!! I thought my wrist was hurting!! after all that i hope this helps someone...!!! madness!!