Dave Kopel has an excellent summary of Florida’s stand your ground law, and how it’s not relevant in the Trayvon Martin case, no matter which narrative you believe.
A forensics firm has taken the 911 audio that’s been released and analyzed them, and determined the screaming voice was not George Zimmerman’s. The guy who did the analysis is credentialed. That said, I’m skeptical of a lot of this CSI stuff, after so much of it that was accepted as scientific was later proven to be based on junk science. Phones and walls acts as filters for audio signals. If I’m on the jury, they need to show me, using the same methods, how much the scream correlated to Martin’s voice before I’ll accept it. Keep in mind there was an eyewitness, and in my view as a juror, an eyewitness is going to trump audio forensics, if they can’t ID the scream to either voice. Also, one person in this field thinks you’d need an exemplar of Zimmerman screaming to do an accurate analysis.