Europeans fought against African Kingdoms, and yet here's the difference between European Slavery and African Slavery:
How did the Europeans justify their enslavement of people? Racism. They made up the crap and even used the BIBLE to say that Europeans were genetically Superior to the Africans, and that they were meant to be slaves, because they are descendents of Ham (One of the sons of Noah, who's son was cursed because Ham "saw the nakedness of his father"). The curse of Ham made it to where all his descendents were evil or something like that. The Europeans used that curse to say that all the Africans (and Black folks) were his descendents. And not to mention, the Europeans treated their slaves quite Harshly, and the many slave rebellions says it all, especially the Haitian Revolution
Now in contrast, African Slavery
Slavery in various forms has existed throughout human history. Africans enslaved people long before Europeans arrived — as did Europeans and Asians.
Africans enslaved by other Africans usually were criminals, debtors or war captives, historians say. African slaves generally had rights that included the right to own property and to marry free people.
In conclusion, European slaves were a piece of property, with no rights. Slavery within Africa was different. A slave might be enslaved in order to pay off a debt or pay for a crime.
That's the difference. Those African Kingdoms didn't go calling their own slaves "[censored for obvious purposes]" or any slurs like that, Europeans did.
And note, even after the end of slavery, Belgium was still doing that in the Congo Free State, and there were still a bunch of Genocides, Herrenvolk democracies (Herrenvolk Democracy - a system of government in which only a specific ethnic group participates in government, while other groups are disenfranchised), Revolutions for Decolonization to be TRULY FREE, Civil Rights Movements to even give these black people equal rights, Civil Wars to Free Slaves (Like for example in the US), and Other things.
Also to note that Haiti, a Slave-lead Nation ended slavery before most of those European Empires did.
https://news.osu.edu/when-europeans-were-slaves--research-suggests-white-slavery-was-much-more-common-than-previously-believed/#:~:text=While%20African%20slaves%20did%20grueling,rowing%20the%20corsair%20galleys%20themselves.
https://www.virginiamercury.com/2019/05/23/fighting-myths-misconceptions-and-misunderstandings-about-race-slavery-and-the-civil-war/