Then his statue should serve as a reminder of his crimes, of our ancestors failure to oppose him and his supporters.

Perhaps it should, I don't know much about the statue or the way the community that hosts it perceives it. My point is only that harsh criticism of Leopold II doesn't rely on retroactive application of modern values. He was a fiend even when judged by the standard of popular contemporary European values: https://en.wikipedia.org/wiki/Casement_Report