Carol was never a cold blooded killer to begin with. She's just someone who's willing to do whatever it takes to protect the group. I think that she always did find it difficult when she killed someone:

- Karen and David at the prison was definitely something that she regrets and the guilt still weighs down on her even up until this point.

- It wasn't easy for her to kill. Lizzie (or was it Mika? I get the sisters mixed up) even though she knew it was necessary (the kid was crazy and too far gone).

-She had to kill everyone at Terminus to save the group.

-She had to kill those Wolves who attacked their community. At the end of that episode, you can already see the damage of all of the killing has done to her. Of her being afraid of what she was capable of becoming/doing.

There was a 2 month gap from when they got rid of the herd in Alexandria and meeting Jesus. And like you mentioned, it has given her a lot of time to just reflect on everything she's done during the ZA. Morgan, as well as the guilt of Sam's death, definitely has an influence on her transformation. It has been gradual but something that they have been building up for a while IMO.