Here's a read for you guys. There has been an international treaty that's existed since the 60's that would ban nuclear testing (I always thought that it was already in effect, though the article says it isn't).
That's one reason why, however, the U.S. and the U.N. and NATO and anything else you can think of is pissed off with North Korea, is that they are performing this tests against the treaty (I can't tell if they signed it, I don't know where Korea is on the map given, in that jumble of smaller counties below China).
So, that's one thing you have against them. Second up, is that any nation with a nuclear warhead can use it as a bargaining chip. For those of you who are paying attention to the news, Japan has become a target apparently for Korea's hostillity, since all of their long range missles get shot towards them. For those of you who know your history, Japan is the only nation to have ever had a nuclear weapon used against them, and happens to be the only world power that does not have nukes.
So, you got another reason to freak out. Any other nation that Korea would want to use their nukes on wouldn't have enough time to set up missle to missle air defenses by the time we figured out who they wanted to target, and setting them up at sea makes N. Korea pissed off (As shown by the recent news story where they said that ships in the waters nearby was a sign of war).
So, everyone in N. Korea's goverment is crazy loony whacko and anyone that could secure a nuke would actually be quite a bit threat..... to somebody.