Of course, historically people "could have imagined". I'm talking about seeing this through the eyes of a civilian that is brought off a train wagon and told they are being put in a labour camp. In that situation, I think very few people have it in them to imagine that their captors are organising the largest and most industrialised mass murder in history, and that they won't even make it out of the "showers" alive.
I don't expect them to launch a revolt, but with prisoners outnumbering guards 100:1, I don't think so many would have walked to their execution in orderly files. I think there would have been a lot more kicking and screaming involved if they knew what was coming. Remember that these weren't strangers either: We're talking about whole families and all their friends sitting calmly together on the train and walking willingly into the gas chambers. That only happens if people are lured into thinking this is something other than it is.