Amnor Sen is nothing if not a paladin, and he is required to do what is both good and lawful. Unlike hellknights though, his law is mutable, generally following the tenets of a god, or, failing that, following the law of the land he is in. So in Geb, a land where undead slavery is not only expected, it is in fact a law that all slaves be undead, he is required to follow the law or risk losing his connection to Shelyn, and becoming a fallen paladin. There is considerable leeway in how he can interpret the law, but the fact of the matter is he cannot allow a runaway slave to go free.