Negan truly sees himself as a savior of mankind to justify his brutal actions against the “tribute” communities, all of this prior to the actual war.
Rick on the other hand knows what he has become and takes action against others that he think will do harm towards his people, regardless of if its true or not. I’m sure Rick regrets Carl not killing the new doctor guy and sees that as a failure of his fatherhood.
No, it was Rick who failed to kill the new doc, Siddiq. Rick shot over Siddiq's head and allowed him to escape. Because he showed mercy, Carl died. This is what "flipped Rick's switch" to cold blooded killer. Of course he will read Carl's letter and that will bring him back. Rick has done this flip a couple times now. Remember "Farmer Rick" back at the prison...