The Belgians did'nt; the Congo was originally a legally independent polity with the King of Belgium as its absolute Monarch and said King is the one responsible for the atrocities in the Congo, not the Belgian state, which for its part immediately passed laws stripping the King of his power and annexing the Congo to try and stop it when the full scope of what was going on became known to Europe.
While the Belgian Congo was less horrific than it had been under the Congo Free State, the Belgians remained terrible colonial overlords, even given the competition. The Belgian approach to colonialism and decolonization were both hideously exploitative and unrealistic ("Wait, we have to plan for independence? Can't we just continue ruling you forever? Fine, we'll give you independence in the 1980s...wait that's too long? Fine, you're independent
now. Too bad no one in the country has any education at all!"
There's a reason most of the former British or French colonies have turned out like Kenya or Senegal (poor, prone to occasional violence, not shining examples of democracy, but not complete and total disaster areas) while the former Belgian colonies have all become the poster-children for post-colonial Africa's worse problems. When the most successful of them is best remembered for having had a notorious genocide less than 20 years ago...