How can we understand all this killing by communists? It is the marriage of an absolutist ideology with the absolute power. Communists believed that they knew the truth, absolutely. They believed that they knew through Marxism what would bring about the greatest human welfare and happiness. And they believed that power, the dictatorship of the proletariat, must be used to tear down the old feudal or capitalist order and rebuild society and culture to realize this utopia. Nothing must stand in the way of its achievement. Government--the Communist Party--was thus above any law. All institutions, cultural norms, traditions, and sentiments were expendable. And the people were as though lumber and bricks, to be used in building the new world.
Constructing this utopia was seen as though a war on poverty, exploitation, imperialism, and inequality. And for the greater good, as in a real war, people are killed. And thus, this war for the communist utopia had its necessary enemy casualties, the clergy, bourgeoisie, capitalists, wreckers, counterrevolutionaries, rightists, tyrants, rich, landlords, and noncombatants that unfortunately got caught in the battle. In a war millions may die, but the cause may be well justified, as in the defeat of Hitler and an utterly racist Nazism. And to many communists, the cause of a communist utopia was such as to justify all the deaths.
The irony of this is that communism in practice, even after decades of total control, did not improve the lot of the average person, but usually made their living conditions worse than before the revolution. It is not by chance that the greatest famines have occurred within the Soviet Union (about 5,000,000 dead during 1921-23 and 7,000,000 from 1932-3) and communist China (about 27,000,000 dead from 1959-61, as mentioned). In total almost 55,000,000 people died in various communist famines and associated diseases, a little over 10,000,000 of them from democidal famine. This is as though the total population of Turkey, Iran, or Thailand had been completely wiped out. And understandably, something like 35,000,000 people fled communist countries as refugees. It is as though the countries of Argentina or Columbia had been totally emptied of all their people. This was an unparalleled vote against the utopian pretensions of Marxism.