Pretending killing is good
Pol Pot was the Prime Minister of the country from 1976 to 1979 and became the de facto leader since 1975. He tried to build an agrarian society. This idea was first sketched out by his colleague Khieu Samphan. Khieu Samphan, while at the Sorbonne University in Paris, in a doctoral thesis in 1959, analyzed the effects of colonial and neocolonial domination in Cambodia. To secure true economic and political independence, Samphan argued that it is necessary to isolate Cambodia and go back to a self-sufficient agricultural economy. In other words independence was considered so important that the Cambodian government was prepared to kill any number of enemies and reduce the whole population to a mediaeval level to achieve it. One of the revolutionary slogan was "we will burn the old grass and new will grow". It's not clear what that new grass would be except mediaeval poverty.
Destroying previous culture
Pol Pot adopted an anti-intellectual policy. He imposed the concept of "Year Zero". The policy Pol Pot adopted was a radical version of Maoism. He believed in Mao's theory that peasants are the true proletariat. "Year Zero" means that civilization should be restarted and all former culture and traditions within the society should be abolished. This concept directed against the urban petite bourgeoisie, intellectuals, politicians, bureaucrats, rich people etc.
Approximately 750,000 to 1.7 million individuals belonging to these classes were systematically eliminated through forced labor. This has been viewed by liberals, anti-Communists and by a certain fraction of Marxists as "genocide". Other Marxists like Mao supported his action because the people he killed are considered "class enemies" of the proletariat, and Mao helped him when Vietnam invaded Cambodia. When Communists seek to justify systematic killing like that we can see that there's something wrong with Communism.