@Strandjunker The killings started with going after Communists, way back to 1919 even.
But gas chambers and concentration camps, started early as well, Aktion T4. The eugenics programme. Hundreds of thousands were murdered, before they canned it and just went with death camps.
It's where they experimented with more efficient means of killing people en masse. Some people did speak out against it. https://en.wikipedia.org/wiki/Aktion_T4