Somebody following the black guy, is no excuse for assaulting the follower. The 6'2 black guy knocks down the 5'2 Zimmerman (hispanic) and proceeds to pound his head into the ground. The police interviewing Zimmerman after the shooting took note of the grass stains on his back, the bloody head wound on the back of his head, and the trauma to his face after the black guy punched him silly. Plus Zimmerman was screaming out for help that wasn't coming.
In this situation, you would be in fear of going unconscious during the beating and possibly being killed. Self-defense is warranted at this point - especially if by shooting the attacker you know you will survive.