This will be argued for the rest of history.
The US military estimated casualties in excess of 1,000,000.
The Japanese government was telling it's civilians that they expected them to defend the nation to the last person, and going so far as to arm them with sharpened bamboo poles in some instances.
So, both sides honestly expected that there would be millions of dead.
From that point of view, the bombs probably saved a couple million lives. Remember that this was the information that was available to the people making the decisions at the time, so that is the information that they acted on.
The reason given for the second bomb was that the Japanese had to know that the atomic bomb wasn't a one time "trick",that the US possessed more than one.
One argument is that it was racist because we chose not to drop it on Europeans. Maybe, but Germany was pretty much defeated by the time the bomb was available for use, so it's a somewhat moot argument.
Another thing to remember is that the actual long term results of the bomb wasn't fully known at the time. There were plenty of theories, but no hard knowledge.
Hindsight changes our view of motives and ethics involved in the decisions made by leaders in our past. It is very difficult to put ourselves in the mindset of those at the time.