I don't think racism factored into this, but if it did, I suspect the one motivated by racism and towards violence was Martin, not Zimmerman.
Initially, in the 911 call, Martin turned towards and approached Zimmerman glaring at him as he did, but seeing/hearing Zimmerman talking to the police, Martin turned and ran around the corner, but then according to the call to his girlfriend he stopped running.
Seeing Zimmerman no longer on the phone, he physically violently assault Zimmerman - a minority Latino on a majority African-American commmunity - and continued assaulting Zimmerman after slugging the chubby Latino minority old guy (to him) down to the ground.
I suspect that while Zimmerman was screaming for someone to help him, Martin was struggling with Zimmerman for the gun, who to that point had not displayed or drawn it. Yet that is one aspect of the struggle that would favor Zimmerman. Heavy people generally are slower and Zimmerman seems not particularly fit, but heavy people are strong from carrying their weight.
At that point I think it became only a question of which one died. If Martin could turn the gun on Zimmerman, Zimmerman dies. If Zimmerman could turn the pistol on Martin, Martin died.
Martin died.
This didn't happen because a civilian watchmen was doing what they do. It happened because Martin wanted to and believed he could beat the crap out of an pudgy older Latino guy who had called the police on him. I don't think race came into it, but if it did, it most likely Martin a racist against Latinos leading him to charge and assault Zimmerman.
Despite all the words claiming Martin was an angelic child with his Skittles, that he attacked Zimmerman and continued assaulting him (according to TWO eye witnesses) shows he is anything but a sweet young 17 year old teen.
He could have just gone home or cursed Zimmerman out. But he could not beat the s...t out of Zimmerman as much and as long as he wanted to. Anyone who knows about fights knows its not like in the movies with no residuals. Fights leave people crippled, brain damaged and dead - whether deliberate or not. That Zimmerman had a wound on the back of his head, not just a slug in the nose, proved the assault of Martin was extremely violent and of life threatening natures.