The sniper had eyes on him. He must've seen him train the gun. He should've never been able to shoulder the rifle, let alone fire between 3-9 shots.
There was a photo a few pages back from the vantage point behind Trump as he stood on stage. The shooter was on the roof of a building in the background. The building the snipers were on seemed to be behind trump, and to the right of the photo. There looked like there were some trees between the snipers and the shooter's roof.
Given that we have been told the shooter's building was a staging area for police, including sharpshooters, I wonder if there were any impediments to the agents/officers on overwatch behind trump - if they could clearly see the target on the rooftop to eliminate him as a "good guy."
I also think about the stories of an officer coming up over the roof edge, having the shooter point the rifle at him, falling/retreating, then the shooter shooting quickly. I wonder how that times up with the images of the overwatch snipers on target, then one of the counter snipers pulling up from the scope like, "WTH is going on?", then back down on his weapon and engaging with the shooter quickly.
It seems, without any overlapping timeline videos, that this was a magnificent cluster right before the shooting (leaving all the other questions aside right now): shooter on the roof, counter-snipers unable to ID, Officer coming over the roof ledge causing further confusion to snipers as shooter swings gun around on rooftop, sniper pulls off magnified scope to get a broader view of the scene, shooter shoots, counters engage. All in a matter of seconds.
The most potentially interesting piece of evidence to me would be a video shot from the sniper's scope, if it was set up to have that technology, which I doubt. That would at least help answer questions regarding the propriety of his actions.