Remember who caused nuclear crisis in East Asia?
Remember what motivated S.Korea to deploy missile defense system?
Who says "South Korea should pay for $1B missile defense system?"
So, it means that Ttrump created the situation to buy American weapons in East Asia to boost military industry in US.
He might have applied his way of business to diplomatic policy. It can be evaluated as a marketing skill, but won't be task of politicians, who has to pursue peace and moral.
Trump: South Korea should pay for $1B missile defense system