Was the nuking of Hiroshima and Nagasaki horrific? Of course.
Should we be proud of the fact that we're the only country to have engaged in nuclear war? Hell the **** no.
Should we feel horrible for the suffering and pain that it caused, both on the victims, survivors, and national psyche of Japan? You'd better believe it.
Was it justified? Damn right it was.
World War II was, beyond any shadow of a doubt, the single most horrific event in human history. It was a truly global affair, with fatalities potentially as high as 75 million and casualties obviously therefore in the hundreds of millions (remember, casualties = killed + wounded + captured + missing). It utterly devastated two continents, wrecked havoc across dozens of countries, and almost wiped out a number of religious and ethnic groups, especially European Jews and the Roma.
Atrocities were committed en masse. From the Bataan death march in the Philippines to the Nazi death camps in Poland to the rape of Nanjing to the firebombing of Tokyo and Dresden to untold numbers of massacres, rapes, and other crimes against humanity. It was a war that sometimes brought out the very best in human nature, but more often the very worst as well. Terms like evil are tossed around far too easily in today's parlance, but the death regimes of Japan and Germany and even the Soviet Union can clearly and easily be described in such term.
But all that being said, despite the horror of the nukings, despite the dramatic impact that it had, despite all the claims that Japan really was on the verge of surrender, let's get the facts straight.
1) Japan was not on the verge of surrender. Sure, it was slowly beginning to be discussed, and many claim that if we removed the abdication requirement of Hirohito that the war could have ended prior to the events in Hiroshima and Nagasaki. This is nothing more that whitewashing the past and looking at the end of the war with 20/20 vision. This is what the Allies knew..... the Japanese became an overly fanatical and warlike country due in large part to the cult of personality that had formed around the mostly powerless emperor. Allied powers initially believed that keeping him in power would, in fact, just push off future conflicts much in the same way that Versailles merely delayed the endgame of WWI in Europe for a generation. Also, note how fanatical the resistance that Japanese soldiers gave on every single island that we battled over. We're talking about battles where 90%+ of the soldiers fought to the death and, in battles on islands with civilian populations, often ended in massacring their own people rather than let them fall into American hands (which a very pervasive propaganda machine led many to believe were raping monsters). Even more, note how even after the nukings were carried out, a portion of the military attempted to overthrow the emperor to prevent his surrender. With all this information in mind, the Allies clearly thought that the Japanese were willing to fight on.
2) With all that in mind, the costs of waging a land invasion of the Japanese mainland were, in short, astronomical. People may like to claim that their estimates were inflated, but that doesn't change the fact that American policy planners really believed that a land invasion would lead to the deaths and woundings of hundreds of thousands of Americans and the deaths of potentially tens of millions of Japanese. In fact, in preparation of the invasion, we began making en masse in preparation for the losses they expected to suffer. To this day, there are still 120,000 WWII era Purple Hearts in stock, despite all the ones that have been awarded in Korea, Vietnam, Iraq, Afghanistan, and elsewhere over the past six decades.
3) Logistically, there would've been only two alternatives to nuking or invading, either increased conventional bombing campaigns or enacting a blockade. Just to give you a reminder, more people died in a single day during the firebombing of Tokyo than died in either atomic blast. And a blockade of Japan would've resulted in mass starvation, as evident by the fact that Japan was already starving by the time that the bombs were dropped and by the fact that thousands of Japanese died from malnutrition-related problems in the weeks after the Occupation began, in spite of the massive amount of foodstuffs and medicine that MacArthur started sending in.
4) Look at the regional theater. China was still suffering losses in huge quantities. The Soviet Union was preparing to (and, indeed, did) invade Japanese Manchukuo (Manchuria). Had it not been for the dual punch of the Soviet invasion and the American nuking, you'd better believe that the Japanese would've tried to hold out even longer, Hirohito in power or not. Additionally, look what happened after the war. The Soviets were prepared to take all of Korea, but they stopped because the Americans asked them to. Do you really think that they would've stopped if America didn't display a willingness to use their exclusive nuclear power? Do you think that Korea would be better off right now if all of Korea was under Northern control? Instead of having one totalitarian regime and one thriving democracy on the peninsula, there'd just be a single totalitarian government. And, with all of Korea, so you think that the Soviets wouldn't have wanted a piece of Japan as well? They could have very well demanded a North and South Japan, which clearly would have worsened the situation for the Japanese up to this day.
So, with all of this in mind, I really don't see how you could possibly say that the nuking of Hiroshima and Nagasaki weren't justified. It's not a matter of what's right. War is almost never like that. What it's a matter of is what is the lesser of two evils. Would you rather have inflicted horrific damage on a portion of a country in exchange for a better chance for peace, prosperity, and overall lower casualties or would you rather have a prolonged war with greater suffering, even more widespread destruction, greater fragmentation, and more potential for open conflicts?