80,000 people died instantly in Hiroshima, with another 60,000 to die days, weeks, months, and even years later from the effects of the radiation burns they suffered that day. In total, it is estimated that 140,000 to 160,000 people died as a direct result of the atomic bomb dropped on Hiroshima.
Another 90,000 would die 3 days later, when the second atomic bomb was dropped on Nagasaki.
The United States War Department estimated that an invasion of the Japanese home islands would result in 500,000 to 1,000,000 American casualties, and at least twice that many Japanese casualties. Ending the war quickly saved 1.5 to 3 million lives, so, in that sense, the atom bombs saved many more lives than they ended.
But, then again, almost everybody who would have died in the invasion of Japan is now dead. Within the next 10 or so years, the entire WWII generation will have passed away. To paraphrase C. S. Lewis, wars do not increase death, since 100% of us will die. All war does is concentrate death into a smaller space and time. But children were born who would not have been, if their parents had died in that invasion, so their lives, too, were, in a sense, spared by the atomic bombing of Hiroshima and Nagasaki.
So, in the light of history and the harsh math that a military planner must use, the atomic bombing of Japan saved a great many lives. But, also, in the harsh reality that, in a post-Genesis-3 world, everyone is under a sentence of death, the end of another war did nothing to stop the death toll that sin exacts.
The fact that all are sinners and that the just punishment for sin is death is the Bad News that makes the Good News so very, very good!