What do I do about my best friend?

Well from what you've said your best friend seems to have a hard life. Having a hard life and all that stress can extremely effect a person in negative ways (aka the smoking, drinking,cutting etc.), it can also make them moody and unpredictable. If you really want to be a true friend to him then help him through the hard times and try to make his future brighter then his present. Try to help him with the drinking, smoking, cutting as best as you can. It wont be an easy process and it wont be over night, but having his best friend leave him all alone is the last thing he needs.