Please. It's not very sensible to even discussa bout Japan and nukes in this context. What happened to Japan is in no way analogous to anything that might happen in near future considering nukes.
Japan was a strong military might, strongest in far east by large margin. US and Japan fought a long and bloody war, and ther ehad been a whole lot of other wars going on in Europe and Africa too (you know, the little conflict called WW2). At the time, Japan was the only remaining axis country to keep on going, even though it was obvious that eventually they would simply lose. Use of the nuclear weapons did bring a short end to what might have been perhaps year or even two more of warfare between US and Japan. I'm not going to start guessing how much worse would've been if the war continued, but in any way it's not comparable to present or future wars. So, in a way it was acceptable to use those two nukes but that doesn't mean that it would be acceptable to use them to end a war today, or in the future. Remember the imaginary France vs Germany match, where France decided to end the war by nuking some German cities... No one would accept that.
Anyway. Originally, the question was why it is so bad for Iran and NK to have nuclear weapons when US, Russia, UK, France, India, Pakistan and China have them?
Apparently the answer is because:
a. they might use them
b. to commit genocide and/or world conquest.
So, what makes it different from current nuclear states? If they have nukes, they must be willing to use them in some situation*. So, a. applies to all nuclear states and thus, ther eis no difference between Iran, NK and current nuclear states.
So, apparently the motive really matters that much. Well, let's take two scenarios and compare results and motives...
1. Iran tries to commit holocaust v.2.0 and nuke Israel. So they kill a whole lot of Israeli citizens with nuclear warheads. A lot of people die, and it doesn't matter to them what the reason for their death was. Israel probably responds with counter-strike, because they are after all very predictable in that sense. A whole lot of Iranese people die because their leaders' genocidal lust for destruction, which some seem to think more or less probable (for the record, I don't, but it doesn't matter in this thought experiment). Not that the reasons matter any bit for them either.
2. US decides that it's necessary to force Kim-Jong Il to surrender his country and let US troops invade, so they nuke some North Korean cities. A lot of North Korean people die, and if Kim is nuts enough and he really has the bomb, he'll probably wipe out Söul in a counter strike. A lot of South Korean people die.
You get the picture? Both scenarios lead to same initial results - a whole lot of people die, and it doens't matter for them why they died. So, it all boils down to whether or not the worthiness of the cause justifies the means. I personally don't think so. In both cases, nuclear weapons invariably draw civilians into the game of politicians. It's sometimes acceptable that soldiers get involved in politicians' schemes, because that's their job and they have accepted it, but nuclear weapons cannot be used without drawing the civilians extensively to the conflict. And regardless of what the actual reason for the conflict is - be it leaders with genocidal ideas or leaders with the idea that every country should be free as long as they obey us - the dead civilians don't care, and surviving civilians probably care even less about the original cause.
So, why would it be less acceptable if Iran used a nuke compared to a possibility where some of the existing nuclear countries used a nuke?
*If the nuclear states would categorically refuse to use nukes in any situation, there would be no need to keep them in storage. Thus, because they do have them on storage, they must think that in some situation it's acceptable to use nukes, which is a profoundly chilling idea IMHO. Even more so because we have no chance of knowing what kind of situation would be deemed acceptable to use nukes in.