I'm going to get in a fight with my ex-best friend because he was self harming himself over my ex-girlfriend
Why do you want to fight with him?
I hate to see him be sad over my ex-girlfriend.
It is understandable to be upset with a friend when he is hurting himself and you cannot do anything about it but you shouldn't fight with him. It could lead him to hurt himself more.
I'm his best friend and its my personal choice to punch him out.
It's never justifiable to use violence of any kind. You should remain calm and talk to your friend. You confronting him is not going to do any good to anyone.