Crimes committed under the leadership of communists, including genocide:
Soviet Union
Mongolia
Stalinist repression in Mongolia
China
Campaign to Suppress Counterrevolutionaries
Tiananmen Square Massacre Cambodia
Ethiopia
Cuba
North Korea
North Korean Concentration Camps
Terrorism
When the goal is a vaguely defined eternal utopia, any evil is permissable on the path to that goal. This is a trait shared both by Communists and ISIS.