Part of the problem is that Kim Jong-Un is relatively young, not well versed in international diplomacy, and because he's new to the top job in North Korea, has to prove to those who would take his place that he's a hard man. He may be gambling that by talking tough and threatening, he'll get concessions from the West. Actually, it's a bit like a Jack Russell terrier yapping at an Irish Wolfhound. He knows, and we know, and he knows that we know, that if he started a serious war, he'd lose. If he started anything nuclear, he lose even quicker. Both the West and China would move very swiftly to snuff out anything as destabilising as nuclear conflict; both have too much to lose by engaging in it, as does Russia.
Still, the brinksmanship makes things tense, and that'll probably continue for a bit. In the end, North Korea will have to eithr back down, or will depose Kim Jong-Un for someone more pragmatic.
