A very great deal of good can come of exploiting others; a whole lot of good - for me. As for them, fuck 'em. They're only <insert rationalisation here>.

Owning a slave pre-industrial revolution doubles your manpower. Owning ten multiplies it by 10.

And BTW slavery still exists, either literally or functionally. Plenty of sweatshops in the world, and I hear chocolate isn't always grown in happy edens.