It could also be that Stalin and Pol Pot used Marxism to control the people and was not the reason for the murder per se.
To be clear marxism, communism and socialism in all of it's various forms are terrible forms of government, but I feel that the murders can be attributed more to a despotic leader than to his political ideology.
I'm not sure.
Stalin, this definatly can be the case, he saw marxism as a tool for control and ideology as a method to impose slavery.
Pol Pot though, is more freaky, looking at his background, his interests, education and everything else.
He was a living breathing communist idealogue who studied, analyzed, crafted and thought deeply about communism, the kind of nut you might find teaching in a university.
Its scary, but to him, the ends were a communist utopia, to the point where he had problems with other communists (sick freak), murder, violence, and slavery were all means and methods for the communist paradise he envisioned (or as I see, the 12th circle of hell).
Looking at these guys, Marx, Engels, Lenin, Stalin, Trotsky (the acadama favorite, fled stalin and murdered), Mao, Pol Pot, Castro, and the list goes on and on, they all have philospical and ideological differences, differences in goals, beliefs, methods, and means.
But they have one thing in commom.
Murder.
It even gets interesting when you look at why China communists didn't get along with Soviet communists, over idiotic ideological differances, these maniacs had no problem with people dying over academic text book definition philosophical differances which had no real world application anyway.