If they're lucky, the DPRK leadership won't be QUITE insane enough to actually go through with it. But Kim-Jong Un is young and inexperienced, and has been thrust to the helm of a military dictatorship with a notorious obsession with projecting strength through force. It's not entirely unlikely that they may be dumb enough to at least attack South Korea, drawing them into a gigantic war that swiftly destroys their leadership and conquers their country. Even China supports the sanctions, so they won't get any support from up north. Hell, they'll be lucky if China doesn't join in on the curbstomp to try and rid themselves of what's essentially a bratty freeloader who tried to rob a convenience store and is demanding that their roommate cover for them when the cops show up.
If they try to attack the US? Well, they may as well just duck and cover under their desks. Launching a nuke at anyone will be bad news for them. Trying to nuke the United States of America? They'll be a scorch mark before we even shoot down their ICBM.