Rhee Syng-man
As president, Rhee took on dictatorial powers even before the Korean War started in 1950. He allowed the internal security force (headed by his right-hand man, Kim Chang-ryong) to put people in prison and torture them if they were thought to be Communists or North Korean agents. His government also directed several massacres, the most famous being on the island of Jeju. This was after an uprising by left-wing groups. While massacres did occur under governments that came after Rhee, there were not as many and less widespread.