Actually, you are incorrect. Japan had already agreed to surrender. The sticking point was that the US wanted unconditional surrender and Japan wanted a guarantee that their emperor would not be treated as a war criminal. Keep in mind the Japanese viewed the emperor as directly descended from a god. Possibly the same as a god -- or as close as you get to it on Earth. And the Japanese agreed everybody who ran the war -- the prime ministers and so on -- could be held and tried as war criminals if desired. Just not the emperor.
We said no and preferred to blast their cities with atomic bombs instead, violating the rules of war (which we actually had also violated very badly with the earlier fire bombing of Tokyo). We killed a huge number of civilians, poisoned the living and the land, and then we decided we didn't want to try the emperor anyhow. Great. So *that* was worth it.