He was a man in great pain, carrying massive guilt about the affair and his betrayal of the doctors.
Pain like that can turn someone into a “monster”.
/knew the backstory long ago
As many have pointed out, “St. Rick” went off and slaughtered Negan’s people in their sleep.
We could all seem monsters to someone else, at some point.
Next season, Negan and Maggie get married, count on it. 😎