The order wasn't to kill him, but to simply make him stop. At the time that it was given, no shots had been fired and no one had been killed. The Imperials were trying to manage and de-escalate the situation, not escalate it, which shooting the town's bell ringer in full view of everyone likely would've done. If the stormtrooper had made it there faster, the bell ringer probably would've surrendered, but it so happened that the bomb went off and Stormtroopers subsequently started firing on the people while he was en route, which is likely why the bell ringer fought back. It was a good order at the time, IMO, but, yeah, he should've sent two stormtroopers instead of just one.