I've been through this twice and it almost always ends the same way. No matter what they tell you, how sorry they are, they go back to what they've already gotten away with. Why? It's a boundary they've gotten used to breaking again and again. It's now a part of the DNA so to speak.
From what you've written, deciding whether to stay or go, if it's based on whether or not leaving may cause him to hurt himself or others, if you try to see it from an arial-view, (taking yourself and the emotional attachments away from the situation) you may be able to see that this is a person that has and is currently
hurting himself as well as others.
Being there doesn't seem to be changing anything so why not reach for a better life/purpose that benefits not only yourself but others you have yet to make contact with