That all can also be true. The reason I see the relationship I see between Negan and Carl somewhat like father and son is because Negan in way cared about him. Negan felt really bad when Carl died. Also during the middle of S08E08 "How It's Gotta Be" he had an emotional moment with Carl when Carl was standing at the gates of Alexandria. I read the comic books after the season finale and from reading the comics if it was someone else Negan would have shot that person.