You mentioned her shooting Wolf-boy being a catalyst. but I think the series has presented Lizzie as the ground zero of Carol's troubles. It was bad enough she lost Sophia to the living dead, but for her own hand to be forced to take the life of a living girl--one of her surrogate daughters in a no-escape situation of tragedy--was too much. She hardly felt human after that (even after Daryl tried to support her during 5A), but she was still intent on protecting her family. Only now, she realizes that taking every conflict to the ultimate end is just a mirror of the same conclusion reached in the Lizzie situation, and that's unbearable.
Sigh..... You and I disagree a LOT when it comes to various aspects of this show but this thing I agree with, almost. I'd put her decision to not kill as happening later than what happened with Lizzie since Carol didn't have this much of a crisis of "faith" (I guess to call it) dealing with the Termites, the Wolves and potentially having to do it with the Alexandrians when they first got there and she put on her Suzie Homemaker routine to gain their trust in case they had to blind-side them if the place wasn't ready to do what was necessary to stay safe. She was also very intent on killing Morgan and it seemed more like that was the tipping point.
When she realized how quickly her mind "went there" it may have begun to sink-in and shock her on where she was at as a person and now that scares her.
The situation with Lizzie was the first step but I think the ultimate culmination of it was dealing with the wolf Morgan captured and her gut reaction to it. (Killing Morgan.)