There are a few ideas to this. I'll discuss the one I'm most familiar with that is discussed in Robert Carmicheal's new book When Clouds Fell From The Sky (unfortunately not available outside of Cambodia as far as I'm aware). Carmicheal seems to subscribe closely to Rithy Panh's (award winning Cambodian filmmaker awarded at Cannes last year for his Khmer Rouge semi documentary The Missing Picture and interviewed S-21 commandant Duch extensively while he was awaiting trial at the ECC) that the Khmer Rouge leadership suffered from a "disease of purity". It allowed them to eliminate their innate humanity as they strive for revolution. Anyone who is not "pure" enough for the revolution was easily eliminated and anyone who is tainted by association was also considered lost and irredeemable, the notion of reeducation as practiced in Vietnam and China was much much weaker in Cambodia. After 1976-77 many of the people who died were actually Khmer Rouge as the revolution started to eat its own. The current prime minister of Cambodia is actually a former Khmer Rouge commander who escaped the Eastern Zone purges by fleeing into Vietnam with his unit and returning when the Vietnamese invaded as part of the invasion force.

The internal logic towards massacring droves of people was driven by an ever increasing paranoia of seeing enemies of the revolution everywhere and an idea that enemies of the revolution were irredeemable, unreformable, and unremitting. The mass starvation was caused by a flat decree that each zone would produce 3 tons of rice per hectare under cultivation. The zone and district leaders knowing that falling short of the mark would likely lead to them being branded enemies of the revolution would short the rations of the people working the land to meet the targets and deal with anyone who didn't meet their own individual targets (planting 50 bags of rice seeds a day, moving x amount of earth) the same way as enemies of the revolution. The central logic of the requirements to be seen as adhering to the ideals of the revolution and the assumption that anyone was not deserved to die and to be "pulled up by the roots" (whole family killed essentially) started driving in on itself in a loop leading to the mass slaughter of Cambodians by the Khmer Rouge.

There were also factional issues within the Khmer Rouge due to variances in ideology and global politics within communism that lead to mass purges especially in the eastern zone (body of a Cambodian mind of a Vietnamese) and northwestern zone (faction of "intellectuals" inside the Khmer Rouge.