Except in its relation to the native Americans, I wouldn't say the US was even close to being the most ruthless great power of the recent past, The US is actually more conflicted, and often more willing to let the intangibles stay its hand, then most in dealing with other nations and peoples.
For true ruthlessness one can look at Britain, who repeatedly implemented policies that predictably starved tens of millions of Indians to death throughout 19th century in order to maximize the profit from the mercantile trade network it carefully imposed on India, and over a span of 200 years made amongst the richest and most productive one fifth of humanity into amongst its most impoverished and femine beset. The fall of china from 1750s to 1937 is nothing compared to the fall of the Indian subcontinent over the same period. As recently as WWII, Winston Churchill consciously ordered policies towards India that starved 10 million of them to death in just 1943 alone, while quiping that there are too many Indians and they breed too fast.
For consciously implementing policies that would lead to the death of millions, Britain in India far surpassed Stalin in Ukraine, or Hitler with European Jews, in terms total number of deaths.