I think R'hllorism will factor into the series overall in two bigways, as both a double-edged sword without a hilt that is used in theNorth and a light of hope held up by the abolitionists in the East.Some of its power is metaphysical in nature, while some of it ispurely philosophical.Focusing specifically on the magic attributed to the red god, it'sworth noting that neither of those exercises of metaphysical powerhave depended on sacrifices. The resurrections and shadow assassinsdepended on, for lack of a more precise term, life force, but mostlythe life forces of Thoros and Stannis respectively, and both of themare still alive after a few rounds. Beric trading his life forCatelyn's was, at most, a willing sacrifice, but that was probablyabout his own life force being already only a fraction of Thoros'life force. By that logic blood sacrifice should work, because it'sstill a transfer of life force, but the evidence for it is prettythin. Melisandre claims that the leeches killed the usurpers, butthat's something her visions let her get out ahead of; she claimsthat the sacrifice right before they left Dragonstone was responsiblefor the favorable winds, but they could have just gotten lucky, andso forth.

And while it does include the life-for-power formula, I’m notsure it is the only belief system that incorporates magic with ablood price. Magic is a force of nature in the universe of the bookswhich by definition eludes conventional understanding, so it could beaccounted for or exploited by any belief system with a comprehensiveworldview. Blood sacrifice specifically isn't tied to the red god,either. Mirri Maz Duur was a follower of the peaceable Great Shepherdbut she at least knew the principles behind this kind of spell,Maester Aemon didn’t get his idea about the power in king’s bloodfrom R’hllorism, etc. The Faith of the Seven seems to be the onlyreligion in the books that doesn’t seem to be tapped into somemetaphysical power or other* and the only one that doesn’t doesn’texplicitly practice human sacrifice. And even then, the trial bycombat ritual is endorsed by the Faith as a means of acquiringrevelation of truth through the shedding of blood. How far can youreally separate the secular practice from the transactional religiousritual?

So to (finally) get around to your ethical question, I thinkthat's one of the big questions that the narrative really asks us.Lives are going to be lost to save the world, in magicalrituals or not. And sometimes they're going to be lost based onchoices that Our Heroes (whoever those heroes end up being) make tosave even more lives. You're not going to save everyone. Jojen,Meera, and Hodor have almost certainly given up their lives to hookBran up to the weirwood power source. And what's an innocentsacrifice? Alester Florent, who committed a secular crime(negotiating with the enemy) in the interests of ending thebloodshed? Was Rattleshirt more or less worthy of the fate that hetook for Mance? Mance is more moral and more useful and plain bettercompany, but Rattleshirt, at least, wasn't accused of breakingWesterosi law, as Mance proudly acknowledged he did. Technically themen of the NW are soldiers and not sacrifices as they're thrown intothe meat grinder against the Others, but it's not like most of therangers who died at the Fist of the First Men had any choice in thematter. The murders of both Jeor Mormont and Jon Snow are more orless the magical thinking of human sacrifice in the real world: fearof the uncontrollable forces of nature and fate and sheer dumb luck,momentarily alleviated by the exercise of ultimate power over anotherperson.

The unnatural tools that the ASOIAF 'verse throws into the mix arelike the natural ones we see in both that world and our own – goodor bad depending on what ends they're used toward, and if they'retempered by good-faith ethical considerations.

*Admittedly this is also debatable; Catelyn and Davos both haveexperiences which seem like their minds playing tricks on them whenthey’re under stress, but which theoretically could be genuinevisions.

Hey there!

Oooof, usually I love hearing how right I am, but I wouldn’t mind being talked out of this one….

Whether or not that particular sacrifice would work doesn’t necessarily tell us anything about king’s blood, right, since Monster does not have king’s blood. There is some suggestion throughout the story that blood sacrifice is a source of magical power, but even that is a tough thing to prove.

As for king’s blood in theory, I was going to say that’s just Melisandre either making it up to push Stannis toward that final sacrifice or making one of her many attribution errors. And my instinct is that it sounds silly. But, turns out it’s not just her and her lackeys:

If Maester Aemon believes it, then there’s something to it.

It’s possible there’s a correlation/causation issue here. There’s a kind of mythic sense to the idea that fire magic is particularly responsive to the blood of the dragon, which is how Maester Aemon is likely to think of kings and possibly what Melisandre senses in the Baratheon line. Or that the idea of “casting long shadows in the world” is less metaphorical than she thinks it is, and shadowbinding is more effective when it’s connected to the life force of someone who has great influence in the world, and she’s wrongly assuming that it’s about “king’s blood” (many kings are influential people so king’s blood might work more often, but that specific title isn’t why it works). Those explanations make more sense to me. Why would R’hllor (R’hllor the personal deity or R’hllor the anthropomorphized representation of fire-related forces of nature) care about Andal inheritance laws? What constitutes a king in Essos, where red priests learn and practice? Khals, ceremonial princes, triarchs, what?

Granted, I don’t know exactly what the sacrifice would necessarily be for, because fire magic resurrections aren’t “only death pays for life,” they are “life force pays for life.” Thoros never used blood magic to resurrect Beric, nor Beric to resurrect Lady Stoneheart. (Not intentionally, anyway, and intent probably does matter for this, otherwise there’s usually power zipping around the atmosphere and never being tapped.) Melisandre does know how to do other spells on her own steam, and the basic theory behind the kiss of life is something that red priests generally seem to know. But Mel, BEING MEL, is almost certainly going to be doing whatever she’s trying to do the hard way.