I was interested to see what happened so I looked it up.
While there were negotiations for an unconditional surrender, the military didn't want this. While publicly calling for a fight to the end, privately they were trying to negotiate a more favourable surrender with the Soviet Union.
Even after the bombs were dropped, the military hard liners didn't want to surrender, and there was even a coup d'etat against the emperor. It took six more days before the emperor announced surrender.
The Japanese however were a spend force by that time, and not really capable of fighting for much longer.
http://en.wikipedia.org/wiki/Surrender_of_Japan
As heinous as the dropping of the bomb was, nothing in war is good. It is a human activity designed purely to kill and hurt as many of the other side as is possible. Decisions made in the context of war can not be isolated and judged as stand alone events in the context of peace.
That all being said- I find it an incredible stretch to compare the dropping of the atomic bomb with Nelson Mandela. As I've said, he was a human who made mistakes, but who is being remembered largely for the positive contribution he made to the peaceful transition to a unity rule. Whatever problems the country still has, they are small compared with the problems which may have occurred if the country had continued going in the direction it had gone.
If we look at the men who achieved great things in recent times, none of them was a saint by any stretch of the imagination. Some of them we only found out later were not what they appeared to be. They were all men with faults, who reacted to what was happening around them- not always in the best way, but leaving the world a better place than it would otherwise have been.
It's possible that Mandela was responsible for the death of some, possibly many. I don't know the facts of that. I do know though that his government of reconciliation achieved something of a unity, even if a fragile one- and probably avoiding the deaths of a great many more such as in the tribal rivalries of Rwanda or the black on white which has all but destroyed the economy of Zimbabwe.