The park was called after Leopold I, the first king of Belgium, to commemorate the 50th anniversary of Belgium. This was in 1877, a decade before the Congo atrocities. (Source: https://nl.m.wikipedia.org/wiki/Leopoldpark_(Brussel) in Dutch)
But I think your remark was valid though. Leopold II's atrocities get very little attention in Belgium compared to Nazism.
There are still quiet a lot of statues of him. Which indeed sounds weird in relation to Hitler statues. It might be an idea to give them the same treatment as the Nazi party grounds in Nuremberg. Proper context next to them.