It doesn't stem from white America's unfair treatment of blacks, or from the fact that their forbears were "slave owners." It stems from the truth that many blacks are possessed of a self-loathing; a deep belief in the inherent unfairness of the fact that they were born black and others were born white.
Rather than do the hard work of coming to grips with their own self-animus, it is far easier and more convenient to simply diisplace that aggression by hating white people.
As long as there are blacks in America, there will be racial hatred. And it will be aimed at whites, merely because they are white.
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