Stalin and Mao were communists. Their ideas were those of Marx and Lenin: dialectical materialism, working class leadership, social ownership of the means of production, planned economy, internationalism.
Pol Pot was not a communist. He himself wrote that his ideas had nothing in common with those of Marx. Because of this, he made many political mistakes in trying to solve Cambodia's problems. But he was not the bloody dictator that propaganda tries to present him as. The United States dropped a lot of bombs on Cambodia. They destroyed an already weak economy and filled the fields with mines and chemicals. There was a severe famine in Cambodia. Plus Cambodia had great internal conflict between the rich cities and the poor countryside.
Collapse, famine, internal conflicts, and a politician who made mistakes. Hence so many deaths. 170 thousands according to the CIA. And yet the main cause of these victims was the bombings, not Pol Pot.
To briefly describe these people would require a large article or a lecture.