This seems pretty simple to me. Since Zimmerman initiated the incident by in effect, without any legal authorization whatsoever, apparently stalking Trayvon, (from Trayvon's point of view), the only person entitled to a self defense claim would be Travon. The fact that Trayvon ended up dead only reinforces this belief.
This self-appointed cop wanna-be should never have been in the position to make this confrontation. The homeowners association of this complex is going to pay millions in a civil suit for their neglect in not getting rid of this loser, whether he ever goes to jail or not.