Come on, look at Kristalnacht. Or the murder of countless Jews openly, either by mob or by SS. They started building the camps because it was taking too long, not to hide what they were up to.
They started building the camps early on, and they weren't about killing people at first, but about controlling political opponents. The "Endlösung" was decided on much later, as were "Vernichtungslager", where many arriving prisoners were killed at once.
What you're talking about isn't ignorance, it's denial. Big difference.
Now, in the occupied nations, I'll grant your point. But those are also the places where the population resisted the SS, typically by hiding the Jews, helping them flee, or just being silent when the SS came knocking. In places where antisemitism was already rampant, the SS had all the local support they needed.
I don't argue that people knew that Jews and other people were discriminated against and shipped off, that was part of everyday life. But that they were sent (mostly) east to be killed in an industrialized manner? No, that was not common knowledge. There was a reason the Wannseekonferenz was secret, and that was in 1942, when the war had been going on for two years already.
While the Nazis themselves documented their killings, there were a lot of euphemisms and codes and even falsifications. They knew that what they were doing was morally wrong, and that it was nothing to give the general population too much information about. There even were propaganda clips showing Jews in relatively nice "internment camps".
It also depended on where you lived. If there was a KZ nearby, you probably knew more than the average citizen.
Another question is if something had changed if everyone had known. Not a lot, probably. Some people thought it was a good idea, most were afraid to speak up and criticize anything. You could be sent to a KZ for simply making fun of Nazism.
