SOMEONE didn't play Black Crusade games!The problem is that as a normal human you can't really get much from chaos. If you are a spacemarine or even primarch, sure. For a normal human if you have the power to get something meanful from chaos, generally it means you can get what you want through other methods anyway. Again being tricked, whether due to the lack of knowledge or being deceived by others, is normal, but willingly join the chaos is just bizzare.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.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.
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.