Atomic bombs are one of the most horrify tools mankind ever created. World war II has seen enough mostrosity even before A-bomb bombing, so, considering how many civilians and cities died before, under bulelts, hunger, deceases and tortures, then A-bombs were nnot so much worse, because I think long military invasion to Japan would lead to the same results, maybe even worse.
Also, I always hear that atomic bombs are one of reasons why there's no any wars between countries who owns them, as well as no other world wars. Even the most crazy military heads or lusted for world domination realizes that if he starts a big war with enemy who has such bombs, no matter if you have your own or not, it could lead to devastatying end for everyone. And I believe it's true.