Once I found out what many people went through as witlesses, I quickly realized why they were bitter. It is similar to what happens when someone falls for a scam that costs them their life savings--they are going to be p***ed.
By being witlesses, they are in a scam (and some had no choice, since they were born into it). This scam costs them any chance of ever having any life savings, their education, any fun while they are growing up, all their time, and often their health. Some lose children because of faulty doctrine or because they die in accidents while out in field circus or because of persecution. Then, they realize that the New Dark Ages is never coming as promised or that it isn't going to be so grand after all--of course, they are going to be p***ed.