I can't say Pol Pot was the purest marxist.
If you ever feel like swimming in a sewer full of slime and garbage, check out the internal debates and divisions between the various factions of marxism.
You'll need a few showers after reading through such sludge, but one thing does become more and more clear.
Marxists are, at heart, egalitarian psychopaths with serious mental issues.
FWIT, Stalin only killed more people then pol pot because he had more people to kill, Pol Pot probably eliminated a larger percentage of his population then Stalin did.
Even the chinese started to think these guys were nuts......and they killed in numbers rivaling stalin.
When I wrote my paper in college about the movie "The killing fields" and ripped on marxism, for what it was, my teacher was quick to use the whole movie to prove her points (as faulty as they were) that America was to blame, and that Pol Pot and his terrorists government were extraordinary deviants of marxism (You should have seen her face when I asked her if he was different because him and his group were educated in france and if the european influence on him was to blame).
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.