I would point to the colonial era, and the processes through which the colonial African farm was created. Africa was largely colonized during the late 19th century, a period when Europe was it's height in terms of economic domination of the world. The colonial state constructed had vastly more firepower, was not dependent upon African merchants or landowners, nor upon the support of ordinary people. Whoever controlled the state could do whatever he or she wanted to, so long as you could keep a loyal coterie of soldiers (often paid by foreign aid money) on your side.

I think it's useful to compare the British Raj in India versus British rule in Sierra Leone. Sierra Leone did not have a history of a centralized state, and people were not used to paying taxes. As a there were a series of of Hut Tax wars, where the government basically waged war to extort taxes on them. The British in India took over an existing revenue system and often reduced taxes. As a result, the colonial state in India always had support by much of the population and elite. However, rule in Sierra Leone always came from the barrel of a gun. The post-colonial states inherited much of the same legacy. Governments in India, Pakistan and Bangladesh always needed a certain level of popular buy in (even when the government was not democratic) and multiple dictatorships have been brought down by popular unrest. Business leaders, community organizers, and labor unions all had levers to pull to keep a government in check. Sierra Leone currently has a vibrant democracy, but previous Sierra Leonine leaders such as Siaka Stevens didn't really need to answer to anyone (except the military), and so he could create a death squad by supplying unemployed street youths drugs, and no one had any levers against the government.

As far as Bokassa is concerned, he was a product of colonial brutality. He is said to have watched French soldiers kill his father by shoving a nail through his head because a bullet was too good for Africans. That's got to mess a person up mentally.

The Postcolonial State in Africa: Fifty Years of Independence, 1960–2010, Crawford Young

The African Colonial State in Comparative Perspective, Crawford Young