Why did the Khmer Rouge exterminate their own people ?

In 1965, while the Vietnam War raged at the Cambodian border, leader Sihanouk distanced himself from the Americans and drew closer to the Vietnamese communists. The United States heavily bombed Cambodia four years later, which allowed the Khmer Rouge guerrilla to strengthen. In 1970, General Lon Nol seized power and forced the downfall of Sihanouk, who aligned himself with the Khmer guerrilla. After 5 years of civil war, the Khmer Rouge entered Phnom Penh, the capital. They established an unprecedented regime of terror that led to the death of a quarter of Cambodia's population. Let's delve into the history of the Khmer Rouge and ponder why this regime was so self-destructive