Thralldom in Orlanthi culture
Orlanthi don't view thralldom as morally wrong. Thus you can own slaves, no one will mind. Except your ancestors maybe, if they decided to adopt newcomers, and not make them thralls. If you go against your ancestors' traditions, you will be punished.
Thralldom differs from slavery in the sense that the children of tralls are born into your clan are free, and grow up to become clansmen.
A thrall sacrifice is worth a 10 cow sacrifice in the eyes of the gods. However, your neighbors' estime of you, as well as your clan's mood will drop: it is a desperate move. While the Orlanthi have no problem with keeping thralls, they don't view sacrificing them too kindly.
If you are very desperate, and your clan's mood is at its lowest, or your clan magic is suffering greatly, then your clansmen might not be upset by a thrall sacrifice.
Also, keep in mind that sacrificing a thrall will make all your surviving thralls nervous and fearful, perfect mix for an insurrection!