How about exploding the bomb in the sky, visible from the emperors palace and the whole city?
Or using it against an actual military target.
The message would have been clear, we can annihilate you if we want and you don't surrender. But they choose to go against the civilian population simply, because they wanted the data, of what a bomb does to a city. And that by that time bombing civilians was already pretty normalized anyway, so trying to save civilian life simply was not an issue.
