So this happened

According to Wikipedia:

Unit 684 was an expendable and unorthodox South Korean "black ops" team whose mission was to kill North Korea's premier Kim Il-sung

The unit was set up by the Chief of Korean Central Intelligence Agency (KCIA) on the orders of President Park Chung-hee. The Defense Ministry said the unit in question was a detachment of the ROK Air Force’s Squadron 2325, which recruited 31 convicted Death Row criminals on April 1, 1968 for a 'deep penetration' mission - one that was essentially a suicide mission, but which promised the convicts/victims their freedom and a new start if they achieved the mission's aim. Faced with the alternative, they accepted, and Unit 684 was born.

Mimicking North Korea's Unit 124, which had earlier failed to assassinate the South Korean President, Unit 684 was made up of 31 men, and the Unit's mission was to assassinate the then-leader of North Korea, Kim Il-sung.

The number of the unit, 684, stands for April 1968 when the unit was founded. The members, being expendable, were harshly trained on the remote deserted island of Silmido in the Yellow Sea, off the coast of Incheon.

After about three years of rigorous training, during which seven died, Unit 684 set off in rubber inflatable boats towards North Korea on their mission. However, en route, it was terminated, and the members forcibly turned back by their instructors, as the Korean governments were now looking towards a peaceful reunification.

In August 1971, the KCIA canceled the mission for good, ordering the instructors to terminate all Unit 684 trainees, and erase all evidence of the project.[citation needed] Before they could be murdered, the Unit 684 members rose up to kill their overseers.[citation needed]

They then attempted to make their way to the Blue House (South Korean President's House) in a rebellion. Most of them, however, committed suicide with hand grenades when roadblocked, and confronted by government troops, before they could cross the Han River.[citation needed] The survivors were sentenced to death by a military tribunal.
-Wikipedia

From the NYT in 2004:

No official data exist. But according to lawmakers who have pushed for compensation for the former spies, more than 7,700 men crossed the border on secret missions from 1953 to 1972. About 5,300 are believed not to have made it back.

A movie based on Unit 684: