If you believe in gods and in life after death, it's not inconceivable that you might consider it an honour to be sacrificed in order to carry a message to the world beyond. Some real-life cultures seem to have taken that view. Frazer gives several examples in The Golden Bough and talks about the sacred king whose reign might well end with a trip to the afterlife. Describing the 16th century customs of the Chichimecas, the friar Marcos de Niza said: "They cast lots to decide who will have the honour of being sacrificed, and with joy they crown him with flowers upon a bed on piles of wood which they set on fire, and so he dies. The victim takes great pleasure in this rite."
"Great pleasure"? Anthropologist Kathryn M Koziol isn't so sure about that. In this paper about another Pre-Columbian culture she says, "Among the Natchez, the mortuary rituals of the elite Suns [chiefs] included the killing of retainers. These retainers were members of the Natchez population and gained prestige for their kin by willingly dying, or at least by performing their willingness to die, to accompany their leaders." My emphasis there.
Professor Nicholas Humphrey gets outraged at the suggestion that the Inca "Ice Maiden" might have been an even enthusiastic participant in her own ritual killing (40 minutes in to this video)...
Players of my gamebook Necklace of Skulls may have been on the receiving end of such an honour. On your visit to Chichen Itza ("Yashuna" in the book) the priests of Chaac invite you to jump into the sacred cenote with a message for the gods. Of course, in the gamebook the cenote undeniably is a gateway into the mythic world. In real life (see photo above; that's me on honeymoon) it's just a dirty great pond in which you'd soon go down for the third time. Pleading hydrophobia was no get-out, as there was always the option of getting sacrificed as an esteemed ball player, which is what the spouting snake-heads represent in this carving:
It's not just in Native American cultures that we encounter the willing (and even apparently eager) sacrifice. Ahmad ibn Fadlan, writing in the 10th century, gives a vivid account of a Viking ship burial in which a female thrall has sex with each of the mourners in turn and is then ritually killed. Wikipedia wonders at the attitude of the thrall herself (a volunteer, according to ibn Fadlan) and says, "While the scholarly consensus assumes that the slave girl would have felt happy and privileged about having sex with many people before being killed, recent work has suggested that we should instead see this as an account of rape and brutal strangulation."
What "recent work" could that possibly be? Time travel? A séance? Because otherwise they just mean "somebody's recent opinion", which is utterly useless. It only tells us about the attitudes of the West in 2021, not of the Rus in 921. We cannot know how people from those far-off cultures regarded a sacrificial victim without knowing what the world looked like through their eyes. Even the word victim implies a modern viewpoint. To them the afterlife was probably as real as sunlight and rain, whereas we know perfectly well they were not off to see Chaac or Odin with a message from the mortal world but were simply being snuffed out. Would they have been frightened or honoured? Both, probably. As Professor Koziol says at the start of that paper I cited, human societies are capable of acts that are simultaneously great and terrible. Our rituals aren't unambiguous, so why should our reactions to them be?
No matter how sure you are that you're about to enter the afterlife, there's got to be a natural fear of death too. A kamikaze pilot crashing into a warship no doubt had feelings of both fear and pride. The September 11 attackers presumably felt exalted, terrified, uncertain and excited all at once. They were told their martyrdom would earn them immediate entry into a heavenly paradise, but did they completely believe that? Would Wikipedia describe them as feeling "happy and privileged" or as having been manipulated and psychologically abused? If we can't even exactly pin down the mentality of modern jihadists, how are we going to judge the cultural mores of five or ten centuries past?
Yet thinking our way into other cultures and other mind-sets is part of roleplaying. On Tekumel, prisoners of war who aren't of glorious enough status to be worth ransoming back to their clans will usually get sacrificed to the gods. In Tetsubo, a character performing seppuku believes he is undertaking an act of courage and dignity. Spock in Wrath of Khan knowingly took a lethal dose of radiation as an act of logical altruism -- it wasn't his fault he was then doomed to an eternal afterlife of sequels, and we can only hope that any player-character attempting the same thing in an SF campaign would be granted the dignity of a true death.
We don't know what it's actually like to be a person raised in another era or culture, but we are all human and we have universal emotions and our imaginations. We can conceive of a credible mindset for a kamikaze pilot or a 10th century slave girl on a pyre or a Maya citizen jumping into a sinkhole. That's as good as it's going to get.