Twentieth-century mass killings in Rwanda and Cambodia were similar in

that

Due to the severity of genocide against certain ethnic groups.

Explanation:

Between 1975 to 1979 between 1.5-2 million Khmer ethnic group members were killed in Cambodia; a quarter of it's population at the time. The Rwandan genocide, also known as the genocide against the Tutsis between 7 April and 15 July 1994 during the Rwandan Civil War. Tutsi, Twa, and moderate Hutu in were targetted.