Slavery is always a hard issue especially when a religion is OK with it. What happens if the slaves are freed? There are very few instances in history where everything works out well in the short term. Someone wants revenge and someone else wants their property back.
The moral grey areas appear when you know that by freeing the slaves you will make things much worse in the immediate future, but hopefully in the long term things will be better.
If the church says slaves are allowed then potentially your seen as the villain for freeing the slaves, perhaps your considered by the faith as a thief for stealing a lords property. A God may even punish those who free slaves.
I don't think other gods would allow someone else to punish their followers. If they did, nobody would worship them. Also it doesn't matter if their "culture" or "religion" deems slavery OK, because the same logic could be applied to murder, rape etc; It's still wrong, no matter who says what.
On-Topic: They said that no known magic can bring a person back, but souls reincarnate into other bodies. Soooo, it rests entirely on the philosophical reasoning of the people/culture. If they don't value a person's corporeal form and only souls are given any significance then I suppose murder won't be such a crime, but on the other hand it still causes a person to cease to exist, even if their souls go to other bodies and I don't think they would have the same personality. We need more info to make even the vaguest speculations.