It’s deceptive for Harry to say “stop being mean to my black girlfriend” without acknowledging that Henry’s ancestors helped build racial hierarchies worldwide.
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How very kind (some might say condescending) of Ms B to consider forgiving Harry for his youthful indiscretions.  How very arrogant of Ms B to withdraw the possibility of her forgiveness due to the action of Harry's forbears.

Were his ancestors involved in the slave trade?  Yes.  Did they support the concept of racial superiority?  Yes.  But so did everyone else in the world at the time.  Many African countries, many non-whites, were willing promoters of the slave trade and gladly sold out their neighbours for cash.  When apartheid ended, there were numerous incidents of abuse against non-blacks.  Does Ms B also refuse to forgive these people?

We should all reject the concept of intergenerational guilt.

And when it comes to the dispensation of forgiveness, who the hell does Ms B think she is?  The Pope?