[throwaaway45656]

In your situation you had no pressure at all. I imagine that even getting to the top of the roof without the police spotting hin is enough to give him an extremely high level of adrenaline that causes his body to start shaking.

In addition he knew that snipers and police were constantly watching roof tops, and if he poked his head out he may not have many seconds to actually raise his gun, aim and shoot.

I am not an expert, but I doubt you can compare your experience on the range to a kid full of adrenaline trying to take a shot at one of the most protected persons in the world. It's a completely different situation.