Good thing you mentioned nazi Germany, because it's actually known figure - 17 milion. That's third place amongst dictatorship countries. Stalin did 23 million, Mao did 74 milion. During colonization actually Belgium was most bloodthirsty with 15 million attributed to Leopold II. From older history Genghis Khan was responsible for about 40 milion.

Recent war on Ukraine is couple hundred thousand in most grave estimates. Afganistan where both US and Russia waged wars lost in total similar number of people. To put things into perspective entire population of Iraq and Afghanistan combined is 83 million.

Then there's case of native Americans which is very hard to assess ranging between 3 and 100 million. But that happened 100 years before USA was established so you have to decide if you want to associate these with USA or European countries.

Considering as anti US stance as possible - about 300 years of US history may more or less equal to 50 years of reign of Mao(though in reality it's probably much lower number). So while they definitely have their sins your original statement doesn't check out from math perspective.

Man that was a few weird paragraphs to write :-/