IOTL Congo Free State villages eventually got to the point where they warred with one another in order to obtain the oh-so-valuable resource of severed hands.
Expand this dystopian nightmare state, have others copy its model, and combine it all with what Jonathan Edelstein said. Might as well call it Generalplan Afrika at that point.
Think of it a bit like
1984's buffer zone - the people are used for forced labor, periodically fought over with no regard for collateral damage, and then rounded up to do forced labor for someone else. I'm willing to bet that African and South Asian bush villages in the 1984-verse fight over severed hands too.
That said, a New Imperialist regime of this type probably wouldn't be a boot stamping on Africa's face forever. This isn't the 1984 universe, after all - it's one in which the colonial powers like to think of themselves as civilized and enlightened, and someone would eventually expose the colonial atrocities and spark an outcry for reform. Also, the spheres of influence would eventually shake out to the point where continued endemic warfare would eat into the profits. So by 1920 or thereabouts, there would be a switch to less lethal forms of resource extraction - but by then, Africa would have experienced a generation of extreme brutalization, and as with the Belgian Congo, even the "reformed" regime would be oppressive.