I dont' see why a normal human would want to "join" the chaos if he has other choices. Being tricked is a more reasonable way storywise.
You base your statement as someone who knows the lore, much of which is not considered common knowledge even among the knowlegable characters, let alone commoners.
And with boons like immortality etc. I can easily imagine even those who do realize the cost may choose to join willingly.
Plenty of other motivations one can think of, like revenge, or another sitation in which it may seem a lesser evil.
Also... Chaos is not that bad.

The game focuses on wars & mutations etc, but these entities are actually more complex and have less brutal & revolting aspects as well. For example, some remote human planet may worship a local god of honour which is actually Khorne, yet they are worshipping a different aspect of him and it just kind of works, without demons or corruption. After all, the immaterium is basically formed by beliefs, thoughts & emotion in Warhammer.