I really get the impression that a lot of slave owners genuinely considered slaves to be "not human" in some way shape or form. Ideals of liberty and freedom were simply not applicable where slaves were concerned. I don't think it was hypocrisy per se, or at least not on any conscious level. It's one of the things that always makes me a bit wary when people complain about political correctness (not that there aren't total loons doing completely daft stuff in the name of political correctness) - our ability not to appreciate that some people who are disadvantaged really don't deserve to be so and are not in a position to do something about it without intervention on the part of the advantaged.