That's amusing.
North Korea is not that stupid.
No actually, it probably is. Un is fooling himself into thinking he can actually take on the U.N. with the threat of nukes. if North Korea Nukes the US, the US and it's allies declare war on NK, or worse, nuke it back.
If they nuke South Korea, South Korea nukes them out of existence. As the SK PM said, if he dares attack the south, his regime will be "erased from the earth"
Russia and China will NOT back NK if it declares war and nukes the US. They know better. China has NO interest in war, it's too risky to their interests with the US. And Russia, similarly, has no reason to want war. The best they can both do is advise NK and make sure Un "cuts that shit out" because they are probably the only (china moreso) ones he will listen to.
TL;DR, They won't attack the south OR the US. They know they will cease to exist if they do. And quite frankly, the rest of the world knows it too.