From what has been reported in local media, the black dude was putting an ass whooping on the other guy when the dude getting whoop drew his piece and fired. If that is true case closed.
The “kid” was a 17-year-old, 6’2” football player. Zimmerman is short and chubby. Martin was on top beating the crap out of Zimmerman. Zimmerman was plausibly in fear of death or serious bodily injury.