I've never believed that Trayvon ran up on Z to throw the "first punch."

[Half-Century Man]

54. I can only think of a single reason why TM attacked first (if thats is what happened)

The only thing that makes sense to me is GZ was close, had his weapon in his hand, and pointed at the ground.
GZ appears to be that stupid. He was certainly untrained. He is mentally/emotionally unfit to be a policeman. He had a small exposure to MMA (martial arts take years of hard work to master), long enough to get the macho aggressive hard sell to keep him a paying customer. He, as a habit, made "unofficial" watch patrols (neighborhood stalker) while armed [question; was he armed when he made official watch patrols?]. When given advice/instructions from an actual professional in law enforcement; he the not-fit-to-become-a-professional-Leo, thought said advice/instructions were substandard and acted with all his not-fit-to-become-a-professional-Leo judgment. And discontinued contact with the law enforcement professionals because....why..they were downing his groove?

He is stupid enough to get close, expose his firearm as not ready, think his magic gun made everyone respect him, and catastrophically underestimate TM.

For his part, TM was followed through his own neighborhood by a adult stranger with a nearly shaven head who stopped him and walked up with a small pistol in his hand, and the asshole had it pointed away. TM probably thought, "I get only one chance"