ONLY time I could accept the officer shooting him is if the cops waving the off-duty cop over before going back to talking to each other and the off-duty cop for whatever reason is walking over with his sidearm drawn and at his side, in which case I could understand the idiot cop thinking a person is trying to sneak up to kill some cops.
However since it is in St Louis 100% chance the cop is just an idiot (50% also racist), well because most of them are from all my experience.