You've raised an interesting point. That point being the practice of slavery, while condemned as barbarically cruel by current standards, was regarded very differently in the distant past. Philosophical notions of personal freedom, equality, and civil rights did not always inhabit our social attitudes but are relatively recent concepts having occurred via generation after generation of civilized progress. In fact, if the human species manages to survive for another few centuries it is fairly certain that historians of the future will look upon many of our current social practices and policies as no less barbaric than slavery. Consider how many contemporary minimum-wage occupations occur as closely similar to enslavement. And consider the barbaric cruelty which is the practice of long term penal confinement for such victimless offenses as narcotics possession and sale, and so on. What seems acceptable today will not seem so tomorrow -- and the same may be said for the practice of slavery back in Yale's time. While he may have been a sonofabitch in humanistic terms what he did was socially acceptable.