Why was the Nazis mean-hearted?

A handful of men with power -including Hitler - believed that the Jewish people were beneath them and the only way to maintain that the German race was the only superior race was to remove others. If you watch the History channel there is a documentary that discusses Hitler's past. It is possible that his grandfather on his father's side - who was unknown - may have been Jewish. This may be a personal issue that Hitler's views began with. It also discusses moments he had as a young man where he was not treated well and the establishments were run by Jewish people.