Oh, I missed that "let us now, assembled noble lords, end this abominable institution through which we have become filthy rich. And my lords, what say ye regarding pushing drugs to the wretches of Asia?"
So you can’t do a good thing just because you do other bad things?
So you must be either a saint (who also did a lit of bad things) or the devil himself (who punishes bad people).
You could just simply acknowledge the fact that Britain was one of the first major powers to abolish slave trade and later slavery across its empire, and it actively pushed other countries to follow.
As they like to say in England, bullocks!