My ex has justed started to cut himself because I left him and it's all my fault should I feel guilty I still care for him should I let him know but I really feel bad because he's hurting himself over me

Ladyprincess... we are all responsible for our own actions.
I don't know the story behind your break-up and in fact it does not even matter... your ex has decided to cut himself and that is HIS decision...
He is probably hurt but it is not a reason to cut himself...
Feeling guilty over other people's actions should not be necessary -unless you do things on purpose...

It is sad that this happens, but it sounds as if your ex needs help from a doctor.

No and No

No, he started cutting hisself because he has a emotional issue and needs professional help, and/or he is doing it as a method of trying to control you and try and force you back ( even a worst ID)
Assuming you are teens, please call and tell his parents, or tell a teacher or couselor, This is the very best thing you can ever do for him, it may even save his life in the long run, This person needs intervention to get the professional help.

If you are both adults, then there is little you can do except let them know that they need help

You did what you thought was right at that point in time... As for him, what he is doing is called emotional blackmail... if you think you decided well to call it quits, then stay where you are or else he'll make you weak. You need to be strong.

Your not responsible for any of his actions. He needs some professional help. The reason he cuts himself were there long before you left and probably before you arrived too. It has nothing to do with you. It is just that recent events have forced his own issues to the surface again.

If you really care maybe you could discuss it with a teacher, counsellor or parent. Or someone else you and him trust if you are a little older.