If the West really cares that much about South Korean civlians, why doesn't North Korea use their threat to extort stuff from the West? Say things like the following:

"Lift the US embargo or we'll kill 15 million South Korean civilians."

"Give us $10 billion a year or we'll kill 15 million people."

"Give us an aircraft carrier or we'll kill 15 million people."

"Give us some better technology or we'll kill 15 million people."

"Give us 10,000 South Koreans to us as slaves every year or we'll kill 15 million of them."

What's stopping North Korea from making these demands?