Pol Pot, Mao Zedong, and Kim Il Sung's leadership killed millions of people throughout the course of their command.  While these leaders have many things in common, it would be tough to say that any one thing could have stopped them.  Perhaps if Marx had not written his ideas of communism, if the Soviet Union did not educate, arm, and use their influence to put these dictators in power, or if the United States had successfully unified the Korean Peninsula at the end of the Korean War, then these atrocities may have been avoided.