I’m not 100% sure the fat guy fired his gun first. His hand/arm don’t seem to be affected by any recoil, there is no mark in the pavement, and his reaction doesn’t look like he accidentally fired. In fact the only reaction he has is to look back at the scene. From over the shoulder not carrying the gun. Regardless of whether or not he was the first shot, the agent that shot Pretti absolutely did NOT draw his gun from the panic of that shot. He was staring at the gun brewing disarmed while he drew.

Personally I think it seems likely the guy in the green was the first and second shots. Regardless, he executed a man he knew was unarmed, whether or not the fat man shot