The accepted “civilized” war practice in that era and region was to be fairly humane to POWs. Hitler in his mad vindictiveness (and/or vindictive madness) wanted to PUNISH the Jews, and many other innocent people, that stood in the way of his evil rage. That ended up hampering his war efforts more than helping them. But this was about far more than mere war. It was about a hellish madness.
There is some speculation that Hitler and some of the other Nazi leaders may have had a little understood disease after the Spanish flu epidemic after WWI. Called a “brain fever”, a generic term used for perhaps dozens of diseases, it hit the same group of young people already devastated by the war and the Spanish flu.
Many of them died outright, however survivors were noted for brain damage resulting in profound rages, accompanied with paranoia and a perpetual state of anger against individuals or groups.
Before Hitler joined the Nazi party, it was a mediocre extremist organization, but his speaking attracted others like him, like moths to a flame. Because he said what they were thinking and feeling.