It was like watching an action replay in slow motion. John saw the punch, saw exactly where it would connect- and couldn't do a damn thing about it.

"Sherlock," he shouted, practically begging, now. "Sherlock. Stop. He's not worth it, come on--"

He had the gun, of course. If Sherlock wouldn't listen, if Garrus wouldn't stop, he could make him stop- but he wasn't about to shoot someone without exhausting the other options first.