Declare Kim Sung Il's threat to pre-emptively strike American soil with nuclear weapons & his blatant uncooperative stance with U.N. Sanctions a willful declaration of war. Authorize black ops / ghost ops strike teams, to actively seek out, then assassinate him & any other of his loyal high ranking officials.
Also authorize any counter terrorism measures in North Korea that are deemed neccessary to find any potential military targets, officials, laboratories, etc that are WMD production capable, and destroy them.
As a followup relief effort, appoint South Korean leadership in his place, and unify Korea with south korean laws / etc. Leave enough of a military prescence neccessary over in North Korea until the transiton is successful. Stop aid to N Korea until this is done.