I don't have references on hand for this right now, but King Leopold was probably the only case of a colonising power not even pretending to have a moral reason for their exploitation. In every other case you can think of from Europe you will find Christianity, development, civilisation given as justifications. Leopold turned the Congo into his own private property without much thought of how other European powers would take it, let alone what the Congolese would think. He was there to amass a fortune and was unembarrassed about that purpose or the methods used to achieve it (massacres, mutilations and the like).

If I can find the references I will, because that makes Belgian colonialism in The Congo a unique case among the other powers that believed or feigned benevolent intentions of one kind or another.