We'd already been doing the firebombing of Tokyo, which killed at least as many civilians as either atomic bomb. The only difference is that it didn't happen all at once. So in that context, there would be no reason not to use the bomb. All you're doing different is saving time (radiation/fallout aside; not sure how well that part was really understood in 1945).
That doesn't change the question of whether any of that was justifiable, but it does put the bomb in perspective a little bit.