For me it was the belief that all people who refuse to become JWs are intrinsically bad. If someone doesn't accept the "truth", NATURALLY it's because "Satan-the-Devil" has hardened their hearts. When you're a dub, you tend to ignore the ramifications of this. Take your unbelieving relatives, for example. You know they're gonna DIE at the big A, but you never dwell much on that, partly due to the fact that you've been indoctrinated to distance yourself from them and they're now virtually relegated to the same status as all the other worldlings, and because the Society tells you that this is the fulfillment of scripture - can't remember the exact one, you know about how friends and family will abandon you, blah, blah, blah...as the Society is wont to do at every opportunity - use the org to fulfill yet another prophecy,etc.
I think back on my childhood now and remember the times we would visit relatives and how nice they were to us, yet we believed at the same time that these very same people who showed us nothing short of love and affection somehow were wicked enough for Jah to want to kill them. The fact that I never thought about that as a dub I now find unbelievable. That is why I find it to be the biggest lie. JW love is conditional. At the same time, any "love" JWs get from worldly relatives is not viewed as true love, and in fact may be the trickery of Satan, etc.
Now, since family has always been of the utmost importance and at the root of every culture, going back to when we were living in caves, I find the most grievious sin the Society has committed is to misrepresent and skewer the meaning of love to the point of dismissing your grandma as unworthy of life. And the loyal dubs never give it a second thought...
Wg