Voicing those doubts, of course, seems like it would have been an excellent way to find yourself next at the sharp end of the priest's knife.
I mean, if we want to be morally objective... what does the war on terror's body count say about the United States relative to the Aztecs? We're 10x as evil?
The type of thinking you espouse can be used to justify virtually any military action, including war crimes. All you'd have to say is "it could have been worse..."
And if you want to make the extraordinary claim of “Imagining the whole space of the alternatives” you should provide some evidence of such. Because right now, most experts who have attempted that have come to the conclusion that Operation Iraqi Freedom was a mistake: not merely one of bad planning or execution, but rather it never should have been attempted to begin with. This includes some of its biggest architects and boosters.
Of course Aztecs didn't see this practice as evil - but why would we care? We have our own moral system, and I don't see any reason not to use it in this case.