If I remember my history correctly, a major point of the gas chambers was that the prisoners were convinced that they had been sent to a labour camp, and were sent into the chambers to shower. By convincing people that they weren't in immediate life threatening danger, it was much easier to control them.
Of course, nobody could even imagine the absolute horror of the Holocaust. If you told me that someone would take hundreds of thousands (millions) of fit, working age people and simply wipe them out, I would have a much easier time believing the other guy that said "no, you're being sent to take a shower before being placed in a labour camp. Life will be hard, but obviously we wouldn't waste resources just killing everyone on the spot."