The situation was completely different, Hitler persecuted the Jews because he hated them, King Leopold II of the Belgians thought that the labor of the masses was one of the most efficient way of working of course, not paying them would diminish the costs. Something he concluded after he had seen how the Egyptians worked on the Suez Canal, a project he was preparing to take over the moment De Lesseps gave up.
Whereas the German economy lay in ruins, the Belgian economy was thriving and Leopold's idea for a colony was one that was opposed by the Liberal Governments he had to deal with. Liberalism of that time did not allow colonization because it was seen as a diminishing of liberty. Not that the Catholic Governments were any better though, they complained about every large project.
When Hitler took office he had a complete, democratic mind you, majority. A majority of people who did what he asked. So when he took office as Chancellor he had only to deal with the president, Ludendorff. But he posed no great threat anymore and died in 1937. With his majority Hitler could abolish the Constitution.
The Second King of the Belgians was tied to the Constitution, in Belgium, as King-Sovereign of the Congo Free State he was free to do whatever he pleased, he had build the government himself.
Leopold began his expedition because he wanted his Kingdom to gain prestige (and some more money for his personal case too, he liked pretty buildings, beautiful women and much food.
Hitler wanted revenge for WWI and thought that the Jews whom he hated since his stay in Vienna, had caused Germany to lose WWI.
Leopold's actions killed people who happened to live there and were seen as uncivilized. Hitler killed people because who they were.