If he wasn’t alone what would shooting him accomplish? You still haven’t actually presented a compelling reason he needed to be kept under a gun.

Once Bushnell was on fire and had stopped moving toward the gate/fence, you are correct, he didn't need to be kept under a gun. However, if he had started to move in a threatening way or if he had been working with a larger group, having the guns drawn could have saved crucial seconds if someone else began to act in a threatening way. The security forces simply didn't know what the fuck was happening, and in that situation, it is better to have the guns drawn and to be ready for the worst case scenario.

I think it’s understandable that people untrained for a situation like this would fall back on the default, I know I wouldn’t know what to do, but calling that “reasonable” as if it really makes sense in hindsight is a stretch.

That's fair. I can get behind calling it "understandable" instead of "reasonable"