It's obvious that we can't all like every single person we meet. Sometimes we meet someone and our hackles rise for no apparent reason. So we just don't like them and might not ever figure out why.
I've had this happen a couple of times in my life. I still work with a lady that I took an instant dislike to. I am respectful to her and we get along but I just do not like her. I finally found a reason and it's nuts but it's all I have. I worked with her mother many years ago and that woman, I did not like and gave her every chance to do right by me but she, over and over, did things that were not in my families best interest. I never went to her office where I might have seen a photo of this lady that I work with now and I never looked at photos on a phone or anything. Nothing. I just figure there is something there.
I think that your boy has taken all he can from this boy. He has been held down in a position where he might not have been able to breathe well and that makes a person feel deeply threatened. He's been bullied by this boy over and over. I'm sure he's had enough and even if he did the scratch on purpose he has had enough.
Please think about what happened and what happens to kids who fight back against their assailants. They get caught and THEY get in trouble. Then they learn that fighting back and defending themselves gets them in trouble.
So they get bullied and feel bad then they do something to stop it and they get in trouble. It's not uncommon for them to give up and just stand there and allow it. Please go to Stand For The Silent and read their boys story.
He took it and took it and took it. Then the one time he fought back he got in trouble. Then he shot himself.
I'm not saying that learning scratching others isn't a needed resolution and getting in trouble isn't needed. What I'm saying is that kids can only take so much and he might be at the end of his rope if he's starting to fight back.