According to this sound guy, there was a 30-ish second period of time that TM was begging and screaming for help. Since he was on top of GZ, the only way I can see him screaming for help is if they were struggling over the gun. (It would be kinda hard for the guy on bottom to be beating up the guy on top such that he couldn't stand up and get away - and since TM had no injuries, I don't think GZ got in too many licks, if any, on him. If TM was screaming, it was because of the gun, IMO).
So, if they were struggling over control of the gun for that period of time, it seems to me that TM's fingerprints would have turned up on the barrel of the weapon or GZ would have some scratches/bruises to his hands or wrists - something to indicate the gun was in play for that length of time and that TM was fighting for his life to gain control of it or deflect GZ's aim. As far as I know, there were no TM prints on the gun, no GZ DNA under his nails, no marks on GZ's hands or wrists to indicate such a violent struggle for the gun.
I just want the truth in this case - whatever that may be. It seems unlikely to me that TM was the one screaming for help for that period of time without some evidence of his having touched the gun or grappled with GZ's hands/wrists in an attempt to wrest it from him or deflect his aim. Also, I think "Help, he's going to shoot me!" or "He's got a gun!" might be more indicative of TM yelling, whereas a sort of generic yelling for help sounds more like a terrified person pinned down and getting their butt kicked to me.
Of course, none of that proves anything. It's just my deductive reasoning for believing it was probably GZ screaming for help. However, that still doesn't tell me who made first physical contact with the other. I'm still on the fence, but some things just make more sense one way than the other.
(I can't put any stock in either family's certainty that each heard their own son on the tape; they both have reason to claim - truthfully or not - that it was.)