Why did Trelawney shoot one of the men who was with Hands?

Israel Hands was headed back to the ship and aiming to get ahold of the powder and bullets left behind. The Captain actually was hoping that Trelawney would be able to shoot Hands, but he missed Hands and hit one of the other men.

"Who's the best shot?" asked the captain.
"Mr. Trelawney, out and away," said I.
"Mr. Trelawney, will you please pick me off one of these men, sir? Hands, if possible," said the captain.
Trelawney was as cool as steel. He looked to the priming of his gun.
"Now," cried the captain, "easy with that gun, sir, or you'll swamp the boat. All hands stand by to trim her when he aims."
The squire raised his gun, the rowing ceased, and we leaned over to the other side to keep the balance, and all was so nicely contrived that we did not ship a drop.
They had the gun, by this time, slewed round upon the swivel, and Hands, who was at the muzzle with the rammer, was in consequence the most exposed. However, we had no luck, for just as Trelawney fired, down he stooped, the ball whistled over him, and it was one of the other four who fell.

Squire was not able to pick a men because the mutineers fired at the same time when squire fired,which made the shot of squire miss