No, it was not justified.
First of all, they were on the losing side anyways, they would have surrendered before it. Second, we didn't have to drop it on Japan itself, we could've dropped it near the harbor and scared them into surrendering once they saw the true power of the bomb. Third, if we had just modified the Potsdam Proclamation(A proclamation that demanded that the Japanese surrendered unconditionally without any say) so that they could've agreed to something. Fourth, many Japanese were killed, yes, if we hadn't dropped it than both sides would have had casualties, but the area they bombed had six civilians for every soldier. More civilians were killed than soldiers in both of those bombings. Fifth, people around the area today still have radiation poisoning and cancer and many other illnesses because of the bomb. I really don't see why we had to drop it there if anywhere. The bomb should not have been dropped.