For me, it was reading books by Russell and Rutherford. The religion in the early days had no resemblance to what I was taught. The most amazing to me was Russell's Pyramid theory.
To me, the fact that some JWs fucked up really didn't bother me that much. After all, Adam, Noah, Abraham, Lot, King David, David's son all messed up big time. So, a little sin in the modern era was no big deal to me.
The intellectual factor was more important to me than the human factor. As I said, I could accept the human failings, but I could not accept the intellectual dishonesty and/or stupidity.