They developed the Congo, and under their rule the Congo had the highest standard of living for blacks.
You are probably referring to the Free State, which had nothing to do with the Belgian state and was the private property of Leopold II. The Europeans who administered the Free State were very loose in their oversight and left most of the work (rubber and ivory mining) to the local auxiliaries (Force Publique), for which they were given quotas. Of course, these negros used these quotas as an excuse to carry out raids and massacre rival tribes. Point being, the "Congo genocide" was merely a mass chimpout inadvertently caused by the Europeans arming their auxiliaries and giving them free rein as long as they met their quotas. After the land was handed over to the Belgian government, the indiscriminate violence and massacres ended as the Belgian government actually enforced law and order.