Or they simply need to believe that Martin attacked him first before he knew there was a gun... and then he goes home a free man.
trayvon didn't attack zimm at the entrance to the park - he actually RAN from him.
For zimm's story to be correct, trayvon has to go from what he described as a kid running from him, into a deadly fighting machine attacking him.
At the same time, zimm would have transformed from a profanity-using angry chaser into a harmless guy out for a stroll, attacked by a man threatening to kill him (although the 911 calls didn't record these supposed threats).
Did BOTH of these things simultaneously happen, just seconds after zimm hangs up the phone?
To me, it's a lot more likely that zimm stayed pissed. He went around complex and intercepted trayvon. At that point - trayvon is a minor that was chased two blocks in the night by an armed man... fuck that, he defended himself, no matter who swung first.... a lot of people would have feared for life and fired upon zimmerman... you just don't chase ppl in the dark like that.