Answer:
Hitler (Or other high-ranking Nazis like Himmler, we still don't know who gave the order) probably realized that if they lost the war (World War 2), the Jewish population would be released. I personally don't think this was the case as it didn't occur to Germany's commanders that they may lose until around Mid-1943, and Hitler in 1945.
Another, more interesting possibility was that Hitler and the High Command were scared of the Jewish minority. Hitler (and by extent the entire branch of German National Socialism) believed in the theory of 'Judeo Bolshevism', in which it was claimed that Judaism invented Communism, and those two things were the biggest threat to the 'Aaryan Master Race'.
One final possibility was just that: some high-ranking Germany literally just hated the Jewish people that much.
We may never know the actual answer. If this helped please mark Smartest. :)