They don't need to use nuclear weapons to inflict millions of casualties.  Seoul is within the range of North Korean artillery.  Within half an hour they would land a shell in every square yard of the city.  They would also be launching missiles with chemical and biological weapons both onto the South and onto Japan.  There would be millions or tens of millions of people dead or injured.  You have to wonder if the people who support war with the North understand this and the fear is that many of them do.