By first withdrawing military forces from the Korean peninsula and letting the two halves decide on their own what the terms of unification should be. American military presence is the main hindrance to peace in that region. Same for Okinawa.
Much of South Korean population lives in the capital Seoul. The city can be enveloped in North Korea’s low-tech saturation artillery fire and the entire population can be liquidated in under half an hour.
Kim Jong Un and his band of Communists should be given a blanket pardon so that the unification process can move forward. They can anyway move to China.