You're making the mistake of assuming morality from your current time, place, and culture is universal morality. You find slavery morally objectionable because the current cultural understanding is that slavery is morally objectionable. Future obedm might find it equally abhorrent that you, for example, routinely consumed the flesh of sentient animals or openly released carbon into the atmosphere for personal gain, or probably a million other things that will be completely unimaginable in polite society 500 years from now.