There were lots of reasons people thought the bomb was a bad idea. One was they thought it was racist because they only did it japan. And a lot of people said it was pointless.
People asked the question would truman of used the bomb on the germans? People thought it was racist because they thought America would only do it to Asians but not Europeans.
Most people found out later on that truman would not just pick a country out and that he was not being racist. Truman said he did it to Japan because they were still strongly fighting. He didn't do it to Germany because he said they were about done fighting. Japan was not preparing to surrender they were preparing to fight till death. Truman also warned japan that he was going to do it if they didn't surrender but they ignored him. But people still thought he was being racist and that he did it on purpose. So maybe he did and maybe he didn't.
Is it ever justified to use weapon of mass destruction? A lot of people think so. It's a quicker and stronger way to end the war. There are minimal losses on the side of the attackers. And it could be used as a threat against other countries that are wanting to start a war. The bombs that were dropped on japan helped not only to end the war but it saved thousands of innocent lives that would have been killed in the invasion.
There are some downsides to it. Like Overwhelming destruction and kills lots of people and in some cases it may kind of be unnecessary.