They didn't convince themselves. They were convinced of it. By the people in power who had the most money to make based off of the exploitation of the Congolese people and their natural resources. Once they were convinced, they had to keep believing it, so that they could sleep at night.
I'm convinced my dog isn't a person, my chickens aren't people and even the worms in my garden aren't people. But I would never treat any of those as brutally as the Belgians treated the Congolese. It is absolutely bey9nf my comprehensionsl.
Many of these atrocities were carried out by their 'fellow' Africans in the employ of the Belgians, though the atrocities were generally ordered (or, gruesomely, regulated) by the Europeans. Sadly, people can seemingly dehumanize anyone, regardless of how they look.