There’s a saying, “the worst thing the white man did to Africa was to come, the second worst was to leave it.”
(Funnily enough, when I searched for that approximate phrase using Google, ‘bout the only hit was an article from FR back in 2003!
“Let Africa Sink”
http://www.freerepublic.com/focus/news/924795/posts
And possibly the best thing the white man did was taking millions of them into the US (even as slaves), else they would have died (from disease, starvation, tribal warfare, Islamic slavery) and none or few black descendants in the US would be alive today.