The situation was over. Everyone had walked away. The teen said the worst things about a person you can say but it was over. He was free to go to his Nazi meetings and Kim was free to go get her botox. Nothing more was going to happen. Then Kim calls Kanye who decides that it has been a few weeks since he punched someone and punches the guy. In the face. This is probably not what Kanye needed to do considering he is already facing assault and battery charges and a reputation for using violence.
I would have much more support for Kanye if he had been next to Kim when it happened and it was a heat of the moment thing. It still wouldn't have been the right move but I would have understood it. However he wasn't even there. He just showed up and walked in and punched the guy.