This thread is probably dead, but the main arguments I hear from people saying it was wrong is that Japan was extremely close to surrender and even negotiating terms before the bombs.
Also that it didn't need to be dropped on a populated city first, and that previous bombing of cities in the war proved that the tactic was relatively ineffective. And that the US had a conflict of interest; they wanted to end the war before the Russians were involved. As well as demonstrate the capability of the bomb to them.
Everything I said is debatable, and I don't blame Truman for making the decision he did. But Orwell's comment, that the arguments involved are "too brutal to bear" is definitely true. Most people can't answer the trolley problem, a hypothetical situation where you have to kill one innocent person to save 5.
Now imagine instead of 1 person it's thousands, and they aren't all just killed instantly but some horribly maimed or irradiated. And that whether it will work wasn't certain, or that the horrible alternative wasn't certain, and that there are all sorts of possible third options, etc. And so whatever you think the best choice is, the arguments involved are extremely brutal.