There is no doubt in my mind that the A-bomb was a horrific "dark death weapon." It's effects and the toll was terrible and terrifying. I am glad that it has never been used again, and I hope it never will.
As a high school history teacher, I have argued both sides- that it was justified and that it was not. I have struggled with this for years, but I believe the decision to use the bomb, and to use it on civilians was the only decision Truman could have made. The battles of Saipan, Iwo Jima and Okinawa demonstrated that the Japanese would continue to fight, and that civillians would continue to die in mass numbers. Hundreds of thousands of Japanese civillians were bombed, burned, strafed and starved before the Atomic bomb was dropped. Yet the government and the emperor did not surrender. The larger parts of the Japanese economy and military lay in ruins- with hundreds of thousands of soldiers and civillians stranded on islands and in China, the Japanese Navy at the bottom of the ocean and the Airforce reduced to suicide attack. Japan was defeated, but would continue to fight on and on and on. There was no sign of giving in, no sign that the slaughter would end. Projections of a million US casualties were made.
As president, what other decision could be made? There were only two bombs-- and so the attack was really a bluff-- the bomb had little tactical value- it would not ease the number of casualties an invasion would cause... and should we have kept firebombing the population night after night until there was nothing left and no one left to surrender? He had to hope that something this aweful would force a surrender. (And remember that the surrender almost didn't happen, a last minute coup by junior officers against the emperor was only unsuccessful by dumb luck)
And I hate this answer because it basically says the end justifies the means- I think this is a poor foundation for ethical reasoning.