While I understand that their testing of nukes is an international statement of defiance, what I don't really follow is how outraged everyone else is about it.

Britain, the US, Russia and China are obviously in control of nuclear arsenals. How is it alright for them to have nuclear defences in place, but not North Korea?

I read that testing nukes is a direct violation of some UN rule, but don't the US still test their weapons in the Nevada desert etc?

Moreover, North Korea are fairly isolated aren't they? Although they have some ties with China and Russia, if they ever dared to attack any of their neighbours, wouldn't they just be obliterated by everyone else? I don't know what they'd have to gain by using one.