I love how emotional the ending is when Negan was listening to Michonne through the walkies-talkie about Carl's letter. Another thing the writers did not fuck up when it comes to character relationship. Carl in a way is like a son to Negan.

To Negan, Carl was like a really nice vintage car to a car buff. Negan is a textbook psychopath. For such people everyone else is an object and an object alone. Carl had some qualities Negan admired, that is, ruthlessness and guts, but that's it. He was ready to kill Carl himself as a punishment to Rick, remember. In that situation, he was more interested in crushing Rick's spirit (and hands) to make him incapable of threatening him ever again. The relationship between Negan and Carl had little in common with a normal relationship between a father and a son.

In an important sense, Negan is non-human. He lacks a whole range of emotional capacity crucial for moral development. Negan is a very cunning individual and a master manipulator of others. It was part of his ability to function in society before the zombie apocalypse. When the zombie apocalypse destroyed civilization, the likes of Negan could finally be themselves without society stepping in and locking them up in prisons or asylums for the criminally insane. If his condition could be reliably diagnosed by genetic testing during the early stages of pregnancy, it would make the world a much better place if every fetus carrying that genotype was aborted.