Both Hitler and Lord Voldemort have had people killed under their leaderships. Both had ordered the killing of humans just like themselves. As you already know, Hitler was a brown haired and eyed man who claimed that blue eyes and blond hair was the master race. And the German Nazi group killed many of the Jewish race thus following Hitler's commands. No one stopped to wonder why a man born in Austria - Hungary with dark hair was ordering the deaths of people with similar features. They were happy to blame someone for their loss in the war. And Hitler gave them one.
Now Voldemort hated a certain kind of wizard. Anyone who wasn't of pure blood was considered inferior. He wasn't a pure blood himself. And like Hitler's case, no one under Voldemort leadership questioned this. Like the Nazi's, they were happy to wipe out those who they deemed inferior. Members of the Nazi group and the Death Eaters simply followed their orders to kill from men who were similar to those they killed.
Hitler started his reign of terror when he got in power over Germany. Voldemort started his a long time before he managed to take over the Ministry of Magic. Hitler wanted all of the Jews gone and thus had most of them sent to concentration camps. And anyone found to be helping anyone of Jewish blood was also punished and sent to a camp or killed. Voldemort didn't bother wasting time with camps. Most of his victims were killed on the spot. And anyone who angered him, mudblood or not, was killed.
Now both of these men also had others killed besides the groups they announced to be inferior. Voldemort made muggle killing into a sport. And Hitler also had blacks and gay people killed. Voldemort rose to power twice. Hitler only once. Voldemort was terrified of death and Hitler killed himself when he was defeated. Both these men were incredibly intelligent men to convince people to kill others who were similar to them. It must have been easier for Voldemort to convince the purebloods to kill the half bloods and muggle borns, the hatred was already there. For Hitler he had to start from scratch to create hatred for the Jewish people. Both managed to cause wars based on hatred .
