I guess I would have to question the willingness of the sacrifice. From what I remember from my Old Testament and Classical Archaeology classes, at least some of the humans to be sacrificed were prisoners of war, and I doubt they were happy with their lives being set apart for the gods. And if someone's told that the only way the city will survive is if the gods get a human sacrifice... well, at least a few folks would want to be a hero.
I think this is really a major part. I'm not sure how things worked elsewhere because the only place I've really learned about it was ancient Mexico, but in Mesoamerica human sacrifice was pretty complex. In a lot of cases, it was an honor. In the Mayan ballgames, sometimes it was the loser (a prisoner chosen for the purpose) who was sacrificed, but in other cases there are images of the winning team or their captain being sacrificed, presumably willingly. The Aztecs had several yearly sacrifices that involved choosing someone to "be" the god for a year, getting the best food, not having to work, being pampered and provided with sexual partners, etc. At the end of the year, they'd be sacrificed. The person chosen was generally quite willing, because the sacrifices were an honor and were also seen as necessary to keep the universe from being destroyed.
That said, those societies also had things like child sacrifice, and they definitely did sacrifice prisoners of war (considered an honor by the winning side, but probably not by the losers). They also expected their upper classes to offer blood in ritual, and some of them sounded pretty painful (dragging a spiked rope through a hole in your tongue? ouch).
But sacrifice was viewed very differently by the people of historical societies than it is now, and I think that's important to remember.
As far as sacrifice in a modern setting, most modern societies not only don't find it ethical but have enacted laws against it, and as a member of one of these societies, I can't see using human sacrifice even if it were suddenly legal. We're all products of our times. I could see substituting bloodletting or a poppet, though, depending on the purpose of the ritual. (I know some people use animals, but I don't think I could do that myself, on a personal level. But I think it's also a valid substitute.)
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