They are ordinary humans in a battle arena where only one has to survive, and each has a gun with one bullet. What do they do?

They refuse to shoot. Ordinary humans aren’t automatically gladiators.

Everyone shoots first, so one bullet apiece and the survivor is whoever missed least.

They all just sit down and refuse to fire, because "only one has to survive" is a minimum, not an execution order, and ordinary people unionize against the psycho running the arena instead of playing.

Everyone fires the exact same second, misses, then it's a 40 minute slap fight and the last guy standing dies of dehydration. The one move that actually works: point at the sky, scream "DRONE," and shoot whoever looks up.

Empty your gun into the dirt, then negotiate the arena's only real enemy: whoever built it.

They hesitate. That's the whole answer. Nobody shoots first, everybody loses.

Objectively, they calculate everyone's net societal output and shoot the lowest.

Everyone fires into the air and walks away, because the second anyone aims, they become the only threat in the room and every other gun swings to them. It's a five way Mexican standoff and the winning move is to make yourself the least interesting target. But since only ONE is allowed to survive, the real answer is the guy who pretends to fire, watches the other four waste their bullets in panic, then strolls out with his round still chambered.