Capitalism in the US produces a lot of corpses, just as British capitalism did before it. But I've literally encountered this argument myself and also seen it in debates. Defenders of US capitalism, with it's 800 military bases, funding of death squads that do such things as smash children to death against rocks, torture programs around the world like the Phoenix Program where people are raped using eels, at least we're only killing Vietnamese or Colombians or Syrians. It's not like Stalin and Mao who had it in for their own people for some reason.
It's a strange argument to my ears and I do wonder if some people just don't feel empathy in an ordinary way, but I want to address it in any case. For the moment I want to set Stalin and Mao aside, except to say that for me while I accept that there were major problems and millions dead this doesn't mean we should accept the full western characterization of the situation blindly. But why is it that US violence doesn't target Americans? Is this due to some sort of benevolence? Lenin has an excerpt from his work "Imperialism: the Highest Stage of Capitalism" that sheds light. In it he recounts a story from Cecil Rhodes, well known for the Rhodes Scholarship program. He was a vicious imperialist and mass murderer. He explained what prompted him to advance imperial adventures.
It's true that US imperialism does not target US citizens. But this is not due to benevolence. This is because it is necessary to have stability at home and stave off civil war by placating the population. Imperialism allows the acquisition of new markets as well as cheap labor and resources. These can be used to provide an OK life for the home population and therefore the home population will not disrupt the system. The exploited countries will of course be chaos. We'll have to have death squads killing children and raping women. But chaos there is not really a threat to the system at home.
But this is only for capitalism in the imperial stage. The earlier stages of capitalism could be very violent to natives. The stage of primitive accumulation, which is the earliest stage, was characterized by extermination of Native Americans and slavery. That's what was needed to advance the profits at that time. Today the US is imperialist, but there are signs that this is waning.