Cambodia is a good example. Under the Communist regime of Pol Pot and the Khmer Rouge the country was turned into the killing fields. Their four-year rule (1975-1979) decimated Cambodia, created widespread human suffering and as many as 2.2 million deaths [1]. I guess this is not the part of Cambodian history you meant but it does show what a Communist regime is capable of given free reign.
[1] https://alphahistory.com/coldwar/cambodia-under-pol-pot/