Slavery happens and slavery is tragic, but keep in mind, its only tragic to people with an ounce of morality and many people do not have any sort of morality and do not see slavery as tragic. Morality is a common feature of humanity though as its its inbuilt by evolution to enable social behavior which increases our chances of survival. Because of this, we like to think of ourselves and our will as sacred due to this inbuilt desire, but objectively, it is not true. However, social instincts are the source of our rejection of slavery, but people should not confuse their emotional rejection of these kinds of things with any sort of valid philosophy. Many libertarians do though and base their arguments on this sort of emotional appeal. Look beyond the emotion into what is true.
There is some grey area in how I see things, like brain dead people, animals, advanced AI (if it ever happens) but maybe I will work through those questions or maybe not. Its an admitted flaw in my viewpoint though.