Probably a little bit of psychological tactic.
To push the South to make the first move?
Kim Jong Un and his ilk (to fortify their tenuous domestic situation and their dire need for Western assistance) are trying to give the Hershey Squirts to the South Koreans (won't work, they are a tough, hardy lot), and also scare visting or stationed civilian foreigners in South Korea (that is a different story, but I am sure South Korea will cool them down and convince them this is Paper Tiger Talk out of Pyongyang).
This little kid Jong Un is all brinksmanship and talk. He cannot back it up with action. He barks like a rabid dog. They have issued at least 6,000 such threats to annihilate Seoul in the last 50 years.
I would just issue a press release from US Forces Korea stating the following: "The Democratic People's Republic of Korea is hereby advised to keep their civilian populations away from the Mausoleums of deceased Kim Il Sung and Kim Jong Il, as they will be targeted and immediately destroyed by various assets in the region, without warning, should there be any military provocation by the North in terms of missile launches, troop incursions, shelling of South Korean territory, or underground nuclear tests, all which will be interpreted as an attack upon the Republic of Korea and therefore an attack upon the United States." (Name the coordinates of the Mausoleums as well as the huge Kim Il Sung and Kim Jong Il gold statues on Mansudae Hill in Pyongyang, and state these will also be completely destroyed.)