In 1975, Pol Pot became the most violent ruler of Cambodia. People started coming up with theories about the Khmer Rouge party, that he wanted to make the whole nation become a hierarchical state made up of peasants and their overlords.
When Pol Pot overthrew Lon Nol for president, one of his actions was to empty Phenom Penh in the capital of Cambodia. Pol Pot took all of the citizens' belongings and crowded them into rural camps. There were ten thousand of these camps. He made everybody from ages 7 and up work in the fields with the crippled and pregnant and if they collapsed, they would be killed on spot. They would work 9 out of 10 days and 12 hours out of a day. They would also starve people that worked in the fields.
Another one of Pol Pot's decrees was that anyone who was educated was their enemy and should be killed. The people that survived said that they hid the fact that they were educated. Thousands of people were executed because of this alone. Pol Pot also killed people from Lon Nol’s government, as well as prisoners and anyone who questioned his leadership.
One of the Phenom Penh high schools was even turned into a prison and an interrogation center. There were sixteen thousand "enemies" sent there and were killed. Only five walked out alive. People would be tied up, have blindfolds put on, and then knelt over a pit to fall to their death. Their massive graves were found after the Vietnamese invasion in 1979.
He virtually destroyed the nation by killing three million people.
[Source: The World's Worst Disasters]