We use body counts as a metric for how bad wars were. On this account, WWII is the worst war, at least of the last few millennia of Western history. On the other hand, when measured by a moral criteria of war, it turns out to be one of the best wars, since it largely satisfies the first three of the four criterion for a just war (though it had significant failures in observing proportional response). As wars go, this is (sadly) about as just as it gets.*
While one can’t treat body counts in war as purely accidental to an evaluation of the evil of a war, the criterion tells us more about population sizes and technological wizardry than about evil as such. We can try to avoid the population-size problem by looking at deaths per size of population, but no one knows exactly how to evaluate the evil of such a ratio (is it worse to wipe out 2% of the American population in 1860 or 85% of a population of a mid-sized African tribe?) and so the temptation to just tally up body counts is hard to avoid. Still, this odometer view of the evil of war comes with significant limitations, which have already been mentioned but develop more fully as:
1.) Body counts as such don’t make wars good or evil. Though it is not an unrelated metric, it’s something like evaluating whether a steak is good by looking at how large it is. Sure, part of the value of a steak is that it’s not too large or too small, and there are some contexts in which it will be called good or bad on its size, but what one means when referring to whether it was good or bad is not a quantitative measure. Likewise, the proper criteria for a war is not bloodiness but justice, and while body count can be an element in this, it can only be so on the basis of a previous evaluation of justice.
2.) It’s more a statement about technology than war. Both the making and the recording of body counts are largely a statement about sophistication in mechanical technique. One has to ship the millions of men to some place, along with massive amounts of materiel, then equip them with tools that are capable of killing with great efficiency, etc. But all this technical wiz-bangery doesn’t change much in the fundamental fact of war. After two governments have specified some swath of ground on which young men are supposed to kill each other, an existential fact has been made to which technology adds very little. In this crucial sense technology and its effect seem peculiarly inept at capturing the reality of war.
*I speak only of the rather limited number of wars that I’m familiar with. Specialists or hobbyists might, for all I know, be more optimistic about the number of wars that meet all four of the criteria for just war.