Let's be perfectly clear: The US was destroying large Japanese cities and their occupants at the same scale as Hiroshima and Nagasaki for some time, just with less efficiency, and much of the Japanese populace was prepared to fight to the death with shovels and sticks.

I'm not saying that the atomic bombs were a good thing; I'm just pointing out that they weren't particularly worse than what the US was already doing, and prepared to continue doing. And that in the moment, a display of such offensive power could be argued to be a quicker way to end the war, and prevent having to do a ground invasion of the home islands. With today's hindsight, we can definitely see clearly the other local and global repercussions of nuclear weapons, which makes the US having used them carry many different connotations.

But that's likely not even the whole reason nuclear bombs were used in 1945. The USSR were only grudingly allied with the US, because they needed help early on in the European theater. Well before the bombs were dropped, the Soviets had ramped up their military strength and were running roughshod over eastern Europe. Germany had already surrendered, and USSR looked towards the east, taking over Manchuria and Korea, with the Korean peninsula split at the 38th parallel at Potsdam, before the Korean War.

The US wanted to use the bomb as a deterrent to the Soviets, and using atomic bombs in Japan in 1945 accomplished that goal, as well as reducing the expense and risk to US military forces already at war, without increasing the effects on the ground very much. Japan's surrender had plenty to do with making the decision on who to surrender to, with the preference being the US and not the USSR. But Japan did not want to surrender unconditionally, they wanted to ensure that the Imperial government could do so while saving face, and probably while not also being imprisoned or killed. It's likely that Japan would have surrendered with or without any atomic bombs, certainly without the second one.

But the US needed to demonstrate to the world, particularly to Stalin, that they could build as many atomic bombs as they wanted, and that came from dropping a second one in quick succession after the first.