After listening to the tape of the screaming I'm not sure who it is but it could be Zimmerman. If it was he was definitely in fear of his life and freaking out. But he also seems like he is prone to overreaction and apparently he has some kind of nerve problems too and is on medication for it. It's unlikely then that he is good under pressure.
But Trayvon could also start screaming like that after he discovers the other guy has a gun and wants to shoot him. I think most 17 year olds would be going hysterical. The tape only lasts about 1 minute between the screams and the gunshot so it easily could be TM screaming at realizing GZ has a gun and is potentially going to shoot him.
But here at 16:00 this really bothers me:
https://www.youtube.com/watch?v=4A5V3T1Iq2I
SH: "Is there anything you regret....do you regret getting out of the car to follow Trayvon that night?
GZ: "No, sir."
SH: "Do you regret that you had a gun that night?"
GZ: "No, sir"
SH: "Do you feel you would still be here for this interview if you did not have that gun?"
GZ: "No, sir."
(GZ then talks about it being God's plan)
SH: "Is there anything you might do differently in retrospect -- now that time has passed a little bit?"
GZ: "No, sir."
That's crazy.
I guess he is afraid what he might say could be used against him but this really makes him seem crazy or like some kind of monster.
It seems so obvious to me that this whole thing was a series of misunderstandings and mutual fear. It's horrible to see someone say something like this and I hope he doesn't mean it. I guess maybe he could be linking it to his earlier God comments but it still seems a bit out.