At the root of communism and socialism is Marxism. Sadly, Marxism has turned into a killing field on a scale we can't comprehend. It's called Democide (when your government kills their own). NOT COUNTING WARS, 262 million people were killed by their government between 1900-1999. Of those 262 million, about 200-240 million were Marxist related (China, Russia, Cambodia, etc.). This includes Nazi Germany. What do you think the NAZI stands for--National Socialist.
With that said, French, British, and Dutch colonialism account for 10 million dead and another 5 or so million were Muslims killing Christians and other ethnic groups. Dictatorships account for much of the rest. The U.S. is not without their blame (Native Americans), but this country has a remarkable record of valuing freedom and life compared to left learning countries which account for about 85% or more of all deaths by government. Historically, leftist ideology is the most murderous teaching the world has ever known!
Source: Statistics of Democide: Genocide and Mass Murder Since 1900 By Rudolph J. Rummel