Even as an excuse to seize Congo from Belgium to add to one's colonial empire, it's still not cause enough for war. The fact is no one cared enough to send soldiers to die for indentured Africans just to put one man to trial. Sure, they did care, but the problem itself is effectively resolved by pressuring Belgium on the world stage into taking the Congo off their king's hands. And if Leopold refuses, the likely answers are still going to be 'owell, at least we tried' or 'ok, King of Congo, we'll get a president instead'. Also, there are other, less flimsy casus belli to go to war for Belgium, like the language divide between the French-speaking Walloons and Dutch-speaking Flemish. Atrocities against dark-skinned colonials that had long been considered 'three-fifth human' or worse aren't going to be factored in.