first of all, you should never believe those who try to justify killing people. how do you know it (the h -bombing) was necessary to save one million lives? were that many americans really at risk? also, might there not be others means, such as telling the japanese about the bomb, and maybe testing it in a visible way out of the way of harm of people.
second, it is unacceptable to kill innocent civilians in order to save soldiers. that is a basic principle of the laws of war.
thrid, the cost of Hiroshima -- and let's not forget Nagasaki -- the U.S dropped not just one but TWO atomic bombs which many believe was doone to experiment on the effects of the two different types of bomb-- i digress-- the cost of the bombing was not simply the lives of those immediately lost, it was the costs of sending the world into an unending nuclear arms race, the COLD WAR and all the millions of dead as a result of that (in Korea, Vietnam, Central and South America, Africa, etc.). Hiroshima and Nagasaki contributed to that situation.
We are not done with the nuclear dilemma. The problem has not been solved, so it is regrettably way TOO early to tally up the actual (and potential) dead and issues conclusions about justification.