“Mini-Holocaust” (“mini” because the number of its victims was 20 to 30 times lower) is by far the most appropriate overall assessment of Aktion T4 (Nazi involuntary euthanasia program).
Indeed, both were a classic democide (genocide is a democide) – extermination of a whole social group (thus its victims were murdered because of who they were – not because of what they did).
Both were considered a “final solution” to a problem allegedly presented by the existence of a certain social group – Jews in the case of a Holocaust and mentally and/or physically sick or deformed individuals in the case of Aktion T4.
In both cases victims were adults and children (of all ages), men and women and came both from Greater Germany and from the occupied territories. The same murder tools – firing squads, stationary and mobile gas chambers were used in both extermination programs.
As well as the facilities of the same type – death camps (miniature in the case of Aktion T4 and gigantic in the commission of the Holocaust). Even many actual murderers were the same – most Aktion T4 perpetrators (government officials and men of the Einsatzgruppen) went on to take a very active part in the Holocaust.
Even the reasons to commit these horrendous crimes were almost identical – to protect “German blood and honor”, to win the existential war, to save food and other valuable resources and to free up space (hospital rooms and beds in the case of Aktion T4 and houses, apartments and office space in the case of the Holocaust).
The only fundamental difference was that Aktion T4 finally generated sufficient protest (both from the “common folks” and from who today would be called “influencers”) while the Holocaust brought about no protest whatsoever.
The reason for this difference was obvious – the Nazis learned their lesson from their defeat. And although secrecy of the Holocaust was not exactly airtight, it was sufficient to ensure that just about everyone in Germany either did not know that mass murders of Jews was taking place or viewed this information as an Allied propaganda that had nothing to do with reality.