Some great answers so far. Just wanted to add a response that attends to something you might hear from liberals.

Liberals often, idealistically, have a rosy picture of capitalism and a somewhat quaint and dreamy view of feudalism and whatever Rome and Greece were. When you point out a problem like slavery, which existed in all those periods, they say something like, 'well that was unfortunate, yes, but it didn't need to happen; capitalism can exist without it.' This can be summarised as a claim that slavery wasn't or isn't historically necessary.

If that's where your question is coming from (you might have heard something like that argument), the answer is that slavery was essential. It can't just be deleted from the historical records. We know that it happened, that it was one part of the development of Rome, feudalism, and capitalism. Maybe in an alternative universe there was a path towards the conditions of today absent all the slavery. But that's not the history we're dealing with.

In this way, we can reject the liberal conception of a sanitary capitalism that is not stained by a history of slavery.