The point is that Strictly following the principles of realpolitik I don't think anybody would have any qualms about using slavery to further their means.
Then there are humanists who will have some very different considerations.
My only problem is that people will use one set of principles to look at the world at one time and then use other principles at another to show something in a particular light. Now you can't have it both ways. Either you use realpolitik consistently or use humanism. Be consistent about it. You can't have it both ways.