What was done to slaves of homo sapiens species in the recent past is being done today with dogs, cats and many other slaves of different species. Have you seen the ads where 'owners' of creatures who escaped offer financial reward to anyone who finds them?
People who are against slavery of members of homo sapiens species show no critical thinking or introspection. Since you grew up in the environment where having pets(slaves of other species) is considered normal and acceptable, you are not against it. But, because you happen to be born in a particular part of history where human slavery is though of as bad, you also join the herd in talking against 'evil' of human slavery. Have you analyzed the issue of animal slavery in a critical way or asked yourself why you believe what you believe? Most importantly, have you asked yourself whether anything(any movement of matter and energy) is objectively right or wrong?