Which may be why Zimmerman's defense didn't invoke SYG.
IMHO, there was not proof beyond a reasonable double that Zimmerman was on top. It's quite possible he was on the bottom, and was legitimately scared for his life. From what I know from the trial, I don't think I'd have convicted either.
On the other hand, from what I know from the trial, I also wouldn't have convicted TM if TM had managed to kill Zimmerman. It's a crappy situation for everyone.