I think the point was supposed to be that Negan was always terrible, but his wife and society kept him in check. He's an asshole and he has rage issues, but in his own way he loves his wife, which is why the fight is, specifically, about Lucille not being able to listen to "their" song, instead of being about some other random thing. With Lucille dead and society gone, there was nothing else to keep the monster in check.
For me, I guess the problem is that I don' see how this helps to make the case for Negan NOW. If he was trash before the zombie apocalypse, not a good person turned bad b