A bit of a rebuttal... As Moffesto pointed out, it was mostly the rich who could afford slaves. And it was the rich who had many slaves. And it was the rich who had many sons who didn't have to work, and have nothing else to occupy their time but drinking and so-forth. Such things easily lead to abuse of slaves.
Also, I think you underestimate man's predilection towards wickedness. We've got people now who abuse black people. Think about how much worse it was back then when blacks were considered property? Blacks were running away for a reason. It wasn't cake and candy. A beaten slave is an obedient slave. Beatings given as an example for the rest of the slaves... Sure, some treated their slaves properly, but I wouldn't say even 50 percent of slave owners did. To them, a slave was merely a cranky machine which needed kicking every once in awhile to make sure it kept working.
As for the second point. Yes, there were black people selling black people as slaves. And yes, that was the majority of slaving. However, black tribes would raid other tribes more often cause the slave trade was so profitable. I don't know why you're trying to make white people look better by dealing more guilt upon the blacks.