While I agree that one primary motive was to get more food, Communist atrocities generally start out with noble ideals, at least on paper. Pol Pot also intended to create an ideal society[1], at whatever cost.
Pol Pot actually intended to kill huge numbers of people and wipe out the cities. He had his own crazy philosophy about a peasant Utopia that had nothing to do with Marxism at all.
The Soviets wanted to increase agricultural yield, but the policies Stalin implemented caused the harvest in 1932 to fall by about 20%. In a country already just barely able to feed itself, that led to famine, not just in Ukraine but across the USSR.
[1] https://en.m.wikipedia.org/wiki/Year_Zero_(political_notion)