Zimmerman was stalking Martin, not in the legal sense of the word but in the hunting sense, which makes him a threatening presence to Martin, the aggressor, a predator. It's Martin who had the benifit of the Stand Your Gound law on his side, not Zimmerman. Even if Martin attacked Zimmerman, which is doubtful but a moot point, he had the SYG law on Martin's side. This is backed up on several points, Zimmerman had no reason to approach Martin other than to provoke a confrontation. Martin had every reason to defend himself from an older man who follows teenage boys around parking lots at night. Quite frankly Zimmerman acted like a molester.
The police wanted Zimmerman to stand down. Zimmerman was not part of any trained, bone-fide neighborhood watch program, he was a "self appointed neighborhood watch captain", well maybe Martin was a "self appointed FBI agent". Zimmerman was not performing a duty or service anyone asked him to perform, or he was trained to perform. He was doing it or his own pleasure. A sporting event.
Two audio forensics experts have independently verified that the voice screaming or help was not Zimmerman's. Since there were only two people there Martin, the supposed aggressor was calling for help. ie. Zimmerman could have walked away safely at any time. Zimmerman caused Martin's death deliberately.