The difference between empire and imperialism is the degree to which the local potentates are answerable and accountable to the emperor.
Where the values of the emperor are strong and temper the authority of the colonial governor, there empire rules and serves as a civilizing force throughout the imperial provinces; where the values of the emperor are weak and the provincial potentates tend toward absolute rule, there imperialism rules and serves as a tool of oppression and exploitation throughout the imperial plantations and possessions. Where the local governors have severed themselves from connection with the emperor and taken over the rule of the plantation and possession, there we have South Africa and the evils of apartheit.
The question is how much more evil is apartheit than the exploitation and oppression of a local population by its own strongman rulers who have been coopted and turned into proxy potentates for the imperialist interests that reside in the seat of the empire?
How much does the ethnic identity of the monied interests matter to the political considerations of emperor versus imperialist?