Of course slavery is certainly a bad thing in the narrow sense that nobody would want to be a slave all else being equal.
But in real life all else never is equal - and the question is what other considerations are in practice allowed to weigh in favour of tolerating slavery.
Modern Leftists pretend to be 100 percent under-all-circumstances against slavery; but in real life they always make an exception when to suppress slavery would entail going against those principles which Leftists hold to be more important - such as the privileged status of some social groups.
Thus slavery has been re-introduced into Britain in the past few decades, to a significant but unmeasured extent (certainly some thousands of slaves, maybe tens of thousands depending on definitions) within protected and privileged ethnic groups; and this has happened unopposed, uninvestigated and almost-entirely unprosecuted by government agencies (although to keep Britain free from slavery would nowadays be very easy indeed; easier than at any time in history); because Leftists operate on the basis that the actual real life slave-holding groups happen to be those who Leftists believe should have privileged status with respect to law and regulations.
(Indeed, laws and regulations are nowadays really only intended to apply to the productive and well-behaved among the native population, and perhaps only fully applicable to the productive and well-behaved adult men among these.)
So, given that Leftists are prepared to tolerate, conceal and protect actual slavery as it exists now in Britain, and of course much more widely and openly in Africa, the Middle East and elsewhere, why are Leftists pretending to be theoretically vehemently-opposed to slavery?
Quite simple: because to claim to be utterly opposed to slavery under and conceivable circumstance enables Leftists to claim absolute and qualitative moral superiority over all previous historical societies which openly had slaves - which is pretty much all recorded human societies them until circa 1800.
That's all there is to it.