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)
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.
That I find kind of odd. His treatment of the locals in the Belgian Congo is sort of the gold standard for imperial oppression.
That is true. I'm Belgian; I can't remember having heard about this e.g. in history classes; it's something I came across later on.
What treatment? Wikipedia says that it became a memorial, but doesn't say what it became a memorial to.
It is in a rather unmaintained state (minimal reinforcement), with relevant information displayed next to and on it and is freely accessible to all.