Why did a black man arrest a fellow black man, put him in chains and give him to the white man to go and sell across the ocean in the 18-19th centuries?

Across the world more people fight with others in the fellow race than fight with people of different groups. Europeans did unspeakable horrors on each other during centuries of wars culminating in the two world wars. China had a deep influence on Japan and share a lot of cultural similarities, but the Japanese still did innumerable war crimes on the Chinese.

The biggest enemy of South Korea is North Korea. The biggest enemy of middle easterners are other middle easterners. The biggest enemy of south asians are other south asians. In the same way the biggest enemy for Africans were/are other Africans.

When one group of Africans won over an another group of Africans the latter were enslaved historically. When the Europeans and Arabs were willing to pay money for these slaves - it made a lot of monetary sense - they both take off your enemy and also pay for your booze.