Why is it so important to you that “almost all cultures” practiced slavery? What does that do for you?
At the risk of repeating myself: “they practiced slavery” isn’t an argument you can just throw at any race or nationality in particular without inflicting massive self-damage.
It doesn’t absolve the inherent evil of the institution. No amount of “well billy and tommy did it too” makes it right, kiddo.
I never claimed that. But I’m still waiting for you to find me a single example of a culture that never practiced slavery.