Question

What pro slavery arguments were developed in the first half of the nineteenth century?

Answer 1

Partly because they felt they were "saving" the slaves by removing them from a supposedly inferior environment, one rife with violence and poverty, perhaps, but also one the slaveowners believed was devoid of Christianity. They actually believed they were doing their Christian duty by enslaving, and overseeing the upbringing, breeding and "spiritual growth" of other people they considered less worthy than they. 

They also justified the practice by reading the Bible itself, where it is written that Jews, Egyptians and Romans all engaged in this practice, and where, if I am not mistaken, it is also written that God looked favorably upon it (as long as certain rules were followed).