In the case of Belgium, Leopold, constrained by a strong, large selectorate behaved in as enlightened a fashion as one could hope for in a Monarch. Assume a near-infinite list of how cool he was. In the case of the Belgian Congo, Leopold was unconstrained by any real selectorate, and proceeded to rape the country and brutalize the people. From the undisputed parts of Wikipedia:
I count this as standard monarch behavior. If constrained by strong oppositional forces that encourage the monarch to benefit the people (large selectorate) then he is likely to be well behaved and effective. If unconstrained, or minimally constrained by a small selectorate, he will be effectively a monster."His harsh regime was directly or indirectly responsible for the death of millions of people"