While Marx surely thought his program was in the best interests of the proletariat, the dictatorship was a dictatorship not a democracy:

"The most effective lever for the achievement of this object is the conquest of political power. With its aid the proletariat can consciously carry out the transformation of a Capitalist into a Communist society. To this transformation, there also corresponds a political transition period, the state of which can be nothing else than a revolutionary Dictatorship of the Proletariat." — Marx, Letter to the German Social Democracy, 1875

The Khmer Rouge was that "political transition period" Marx spoke of.