Why try to pretty it up? Why not just accept that the culture your characters are playing in
IS evil and that the people involved have their reasons--which seem good to them--and that everyone pretty much accepts it as their way of life.
There's not much point in exulting in the "shades of grey" or the amount of "relativism" in your world if you insist on prettying things up so that the evil the PCs may compromise with isn't really
that evil.
Of course, it's other posters who are boasting about how they've moved "beyond" "simple-minded" ideas like good and evil. (And then prettying up their "evil" so that it's not really that bad). You were asking how not to make the culture inherently evil. A good way to start would be to ask what would it mean for a culture
inherently evil. Would it mean that every participant in the culture is evil? Probably not. That the culture has absolutely no redeeming qualities? That's hard to believe. A culture would be hard pressed to survive without a significant number of redeeming qualities. Would it mean that the culture is evil with no hope for gradual change? That the evil in the culture is so engrained that removing it would render the culture unrecognizable? But if change did come, presumably it wouldn't come overnight. So, there would be an possibility for good people to live in an inherently evil culture.
So, the next step would be to ask, what role good people might play in a culture that engages in evil practices. And in truth, they can play a lot of roles.
If you like the setting but either you or your players don't want to explore the living in an evil culture aspect, you can let human sacrifice and other evil practices go on in the background. Maybe the characters don't believe in them or personally oppose them, but they can't do anything about it. It would be like setting a police drama in the homocide division of Stalinist Russia. Sure, there are purges going on and that's wrong but there's nothing the PCs can do about it so they just keep their heads down. In the mean time, there are still real murderers and it's really still good to catch them, so they do the good that's available to them and try to avoid being pulled into the apparatus of the purges. (Or maybe the purges just don't happen in their city--in your case, the sacrifices of captives are all made to the emperor in the capital city and that's a long way away so it's all off-screen).
If you and your players are interested in exploring the themes of cultural transformation, you could put them in the situation of ante-bellum Southerners who opposed slavery but wanted it to end gradually. In addition to whatever else the PCs do, they can try to convince people that the sacrifices don't ensure that the sun rises; instead they ensure that the people are always at war with their neighbors and that they never enjoy the peace to reflect upon the ways that they're exploited by their rulers. You could place them in the forefront of a revolution if you wanted to. You could also refuse that opportunity. They convince people who convince people until, sooner or later, a ruler arises who shares their belief. Or you could have a different group begin a rebellion for different reasons. The PCs could join in (possibly only to replace one bloody-handed tyrant with another) or they could sieze the opportunity to open a third front dedicated to the creation of a more just society.
And if you wanted to make things more complicated, the oppressed neighboring peoples or slaves could seize the opportunity to revolt and put all of their oppressors to the sword. In that case, those that the players naturally sympathize with (western society tends to revere victims) are dedicated to their extermination so the PCs can't join them (in fact, they have to oppose them or die). And if this new revolt is simply dedicated to upending the social order and reversing old or instituting a new oppression (as most revolts tend to be) they would have to fight both sides if they wanted to make a different society.
Wombat said:
(Slightly OT -- how does one present human sacrifice in a game without making the whole culture inherently evil? thoughts? opinions? corn-fed chihuahuas?)