And there you have it. An equating of 'liberalism' with 'Marxism'. If that is really what you believe, then there's no arguing with you, as you're coming from a position that is demonstrably and provably false, despite certain sections of the American right holding it.
As for deflecting culpability for the slave trade by putting all the blame on local tribal rulers ... yes, you can safely argue those rulers don't come out smelling of roses but you're not asking WHY they handed over rival tribesmen to the slavers : if the slavers hadn't rocked up on the West African coasts offering goodies in exchange for bodies, there wouldn't have been a slave trade in the first place.
When you talk about modern Africa, e.g. the genocide in Rwanda, yes that was unspeakably awful, but what point are you making? That people of African origin commit crimes or even atrocities? Of course. Human nature good and bad doesn't change with skin colour. But when you look at the so-called civilised West, the preponderance of white on black violence dwarfs any other kind. And when you look at Africa, where is the generalised racial violence of black on white? Yes, it's there, but it isn't institutionalised as it is in America and, to a lesser extent, in Europe.