I'm not disagreeing with you. You can't prove that capitalism was the cause of these deaths. But I do think the most interesting aspect of the concept is that capitalism was by and large the economic environment where these deaths took place.
I'm sorry...but how exactly was WW II with Germany or Japan "by and large the economic environment where these deaths took place"? The war was started by Germany, a country that had already invaded other nations, set up terrible internment camps...and was not in any way a "capitalist" economy? Others didn't go to war because of Capitalism, they went to war because they were being threatened. Which is something that pretty much
any nation would have done in the same situation,
regardless of their economic system (as demonstrated by the fact that both Russia and China, Communist nations, also went to war). In the case of Japan, it is an even more untenable position, since the main countries it attacked (before attacking the U.S.) were entirely non-capitalist economies. And the largest number of people killed in WW II (in Russia and in China) were citizens of non-capitalist states, killed by non-capitalist aggressors. How does this represent a "largely capitalist environment"?
Saying that capitalism was the environment in which these things happened is meaningless, unless some causational link can be provided to demonstrate relevance. Otherwise, it is nothing more than a coincidence. I might as well argue that since the vast majority of those who fought in those wars drank milk, that therefore consumption of dairy products "was by and large the dietary environment in which these conflicts took place".
Also, I think that some people are confusing capitalist with democratic (a common confusion of terms). One can have a non-democratic capitalist state (China's pretty close to this now), and a democratic non-capitalist state (although I can't think of any concrete examples right now, it is theoretically possible). A great many of the wars we are talking about were issues of
democracy vs. totalitarianism, not capitalism vs. totalitarianism. Canadians, for example, weren't fighting alongside their European allies in order to ensure everyone had a chance to make a buck; they were fighting to ensure their continued freedom.