The most scary thing that happened during the holocaust was that Nazis didn’t just use gas. They used the human lives as cruel science experiments. Records exist of them attempting to kill people in very nasty ways with electricity before choosing gas as it was easier.
In the camp of Belzec, in Poland, there have been stories of a room with a metal floor. Nazis would flood the floor with water and force people in. Then they switched on a severe electric current that went through the water and the metal. They stopped because of a current blowing the local power grid. For the Nazis, it was not a moral issue, but a technical one that they could cover up.
Electricity also killed people on the camps of the fences such as Auschwitz. Those barbed wire fences carried 6000 volts or so. Guards used them to put prisoners to death by pushing them into the wires or by drawing them near while they were shooting. Prisoners talked of these deaths being going to the wire.
And the worst thing was the doctor experiments. Doctors like Friedrich Mennecke shocked prisoners off with high voltage to see how much heart the human heart could take before stopping. They also had hand cranked generators to shock people during the process of questioning.
It is disturbing that the Nazis discontinued the use of electricity only because the gas was easier to use. It shows that they were not only murderers, but wanted the most efficient method of killing.