Zimmerman was on public property. He was a neighborhood-watchmen. He wasn't hurting nobody (not even Trayvon) by watching him. If Trayvon didn't like it, he could simply leave.
No altercation is justified.
If it is true that Trayvon tried to grab Zimmerman's gun, then the results (the fatality) are justified.
It all comes down the the screams for help. If it was Trayvon, I am baffled as to how Zimmerman isn't facing a murder 1 charge. If it was Zimmerman, why is he facing any charge at all?