Arguably the two world wars of the 20th century were a consequence of the expansionist tendencies of capitalist economies, and between them deaths were nigh on 100 million.

Of course you may well reject such an argument, as I might reject the argument that the tens of millions of deaths in totalitarian states that described themselves as communist were an inevitable consequence of their politico-economic structure.

My point is not to claim that either of these points of view are true or false. My point is that complex, somewhat subjective and potentially contentious historical analyses are required to attribute causation to the 20th century's multiple clusters of mass homicide (unlike, say, the deaths caused by the 1918 flu pandemic where an objective attribution of cause is relatively straightforward).

The evidence is not as straightforward as you portray it.

(edit: I missed out the word 'deaths' in the first paragraph)