I'll accept your conceit but answer this: why didn't he liberate his slaves? If it's a wash, why keep humans in slavery at all?
I think it's because, in his own calculus, he believed he needed to desecrate human beings, destroy their families, and humiliate their dignity to maintain his greatness.
He did liberate his slaves, eventually. And one of the last things he did before he died was to formally petition Congress to abandon slavery.
I don't think you really have any basis for that analysis of franklins psychology. If anything, I think the safe assumption is just that for most of his life slavery was there and people don't always question every part of society.