Not a good idea. First, it's illegal. If we strike first, we're the bad guys. Second, we can't afford the international ill-will such a strike would create. All those deaths would be blamed squarely on us. Third, attacking NK might provoke a war with China, which wants to keep NK as a buffer state. War with them would be catastrophic. I don't think Russia would become openly involved, but they would take whatever advantage they could from the situation. Europe would not support us.
Basically, there's nothing positive to be gained from a pre-emptive strike.