One pertinent point is often left out of the shocking history of the WWII genocide.
The fact that organised killing of people deemed not worthy of life started with disabled children in October 1939.
Known after the conflict as
Aktion T-4, it even continued after VE day, the last recorded killing being 29th May 1945.
Many techniques for mass-murder were trialled and researched on disabled people, going from lethal injections to carbon monoxide poisoning to gas chambers. Some larger asylum-hospitals had crematoria installed.
The results directly influenced the methods used at later death camps.
Here is a poster from late 1930's Germany:
EuthanasiePropaganda
Nazi party / Public domain
The text reads
"60,000 Reichsmark is what this person suffering from a hereditary illness costs the People's community during his lifetime. Fellow citizen, that is your money too."
I have heard similar comments in my own lifetime, and I always wonder if the speaker should find themselves suddenly disabled (as can happen to anyone, anywhere at any time) or have a disabled child if they would think of the financial cost as the prime issue.
There is a good wikipedia article here (warning, it is disturbing)
https://en.wikipedia.org/wiki/Aktion_T4