This is important to understand.

The Belgians basically levied a tax on the population to produce rubber, and to enforce it, they hired African slavers, who after the slave trade ended, continued using brutal practices to extract as much wealth for themselves as they could.

The problem was really just how cruel people could be to other people. I don't think the Europeans actually understood just how bad things had gotten. They just cared that the rubber kept coming to fuel rapid industrialization in Europe.