Well that leaves the Romulans, the Holy Order of the Kinshaya, and the Tholians out of the group of beings that treat others in a humanitarian way.^Conquest doesn't necessarily equate to slavery. There's a difference between being a subject and being a slave. And as I said before, many armies throughout history have consisted largely or predominantly of troops from conquered nations. Sometimes they've been cannon fodder impressed into service, but in empires that ran things more intelligently, they've been treated fairly well and were given privileges and status to motivate them to serve willingly.