Here are a few examples off the top of my head:
https://en.wikipedia.org/wiki/Atlantic_slave_trade
https://en.wikipedia.org/wiki/Slavery_in_the_United_States
https://en.wikipedia.org/wiki/The_Holocaust
https://en.wikipedia.org/wiki/Battle_of_Blair_Mountain
https://en.wikipedia.org/wiki/Tulsa_race_massacre
https://en.wikipedia.org/wiki/1985_MOVE_bombing
https://en.wikipedia.org/wiki/Bengal_famine_of_1943
https://en.wikipedia.org/wiki/United_States_war_crimes
https://en.wikipedia.org/wiki/United_States_involvement_in_r...
https://en.wikipedia.org/wiki/Police_brutality_in_the_United...
https://en.wikipedia.org/wiki/Incarceration_in_the_United_St...
* Though I will object that it is unfair to attribute the actions of the Khmer Rouge to socialism. Like the Nazis they were socialist in name only, and in fact were supported by the United States in their war against the Socialist Republic of Vietnam.