Human sacrifice is a staple of fantasy adventures. It usually takes place in the sinister temple to a psychopathic elder god, where shaven, skull-faced priest cackle as they slaughter hundreds of innocent virgins. At least, they would if the hero didn't turn up just in time to put a permanent stop to the disgusting ritual.
Now, there's nothing wrong with using this trope. I've used human sacrifice as something practiced by the acolytes of the power of evil, where it's suggested to true offering to the evil god is the worshippers giving up their humanity. But does this really reflect human sacrifice in the real world?
Well, it depends, of course. It's tempting to see all societies that perform blood sacrifices, human or not, as evil, but often that's missing the point. When the Greeks sacrificed animals to the gods, for instance, all they were really doing was slaughtering the animal as normal, but in a special place. A portion was set aside for the god, and then the people had a feast. As simple as that — there was really no difference from a butcher slaughtering the animal purely for meat. We might wonder at the concept of gods who enjoyed the blood, but there was nothing unusually cruel.
When it comes to human sacrifice, there certainly have been societies whose religions wade in blood, but they're in the minority. Roman authors accused the druids of human sacrifice; and, although this can be put down to propaganda justifying the conquest of Britain for entirely economic reasons, some archaeological evidence has been found for it.
"Some" being the operative word. A couple of cases, over centuries and over the whole of Britain, and it's only speculation that these were sacrifices rather than, say, ritual executions for sacrilege.
This is true of the majority of societies that have had human sacrifice — it was rare, and possibly only happened in special circumstances. Imagine a people constantly on the edge of extinction if the harvest fails. There's been a drought for three years — the gods are angry, and if they don't relent, everyone will die. These people genuinely believe that they can send a messenger to the gods to plead their case.
Why wouldn't they do it? It's quite likely the victim would be a volunteer, proud to be the people's saviour. In any case, what society isn't willing to "sacrifice" its individual members — often in numbers that would flabbergast the Aztecs — when there's danger to the whole? We call it war, and that somehow makes it different, but it's the same thing.
It's easy to look at another society and condemn it for what looks like barbarism. Those Roman writers who were so shocked at the druids performing human sacrifice, perhaps in great need, would have happily gone and watched human beings slaughtered in the arena in the name of entertainment.
And what about the tradition of the summer king being sacrificed annually and ploughed into the soil? Well, maybe it happened, maybe it didn't. There's always a problem when outsiders try to understand a religion's descriptions of its rituals, because most religions tend to describe them in very symbolic terms. Many Romans (again) were deeply shocked by the early Christians because it was "well known" their central ritual involved eating human flesh and drinking human blood. In the modern west, even if we're not Christians, we instinctively understand the symbolism involved in this, but it wasn't so obvious to the Romans.
So maybe the king was sacrificed annually, or maybe it was never anything but a symbolic ritual. Maybe it was real in some places and symbolic in others — we'll probably never know for certain.
I hope, though, this has made you think, as it's made me. We don't necessarily have to like customs and religious practices in other societies, and we can certainly be glad we don't have some of them, but they usually make sense in context. And maybe they'd be just as shocked and disgusted at us. It's always worth remembering that.