The Walking Dead has never spent much time on its characters' lives before they were dropped into the zombie apocalypse, which left quite a lot of ground uncovered. The people they were before the world fell apart sometimes help to inform the people they became in the aftermath. For instance, someone like Negan, who is so obsessed with control and enforcing it with maximum cruelty, the person he was in the past may reveal just what made him that way. So what is Negan's backstory on The Walking Dead?
While there's no guarantee the show will stick to Negan's background from the comics — or even introduce it at all — there is a short comics series that explores his prior life called Here's Negan. It followed Negan's life before the outbreak and immediately after it, revealing key moments in the early days of the apocalypse. The series even gave Lucille an origin story, in a way, by explaining just how Negan came to name his favorite weapon. It's clear Negan's comics backstory is intended to garner some kind of sympathy, but it's up to you whether or not you're swayed by his sob story. But before the zombie virus broke out, Negan wasn't all that different from the man he is now.
Negan was a coach at a school who was incredibly inappropriate with his students, to the point that parents were complaining and he may have been in danger of losing his job. He humiliated them using his characteristically foul language and did it in part because he wanted them to think he was cool. He was also juggling multiple women, with both a wife and a mistress, though he chose to end things with his mistress upon finding out his wife had cancer. How gallant.
He chose to be there for his wife during her worsening illness, though she had the rather unrealistic reaction of wanting him to go off with his healthy mistress, and he was in the hospital with her when the zombie outbreak happened. Much like Rick, the new chapter of his life began in a hospital. He refused to leave his wife even as everyone fled, but then she died and turned, falling from her bed in the process. That was when Negan called out her name: Lucille. So, he honored the dead wife he spent years cheating on by naming a murder weapon after her. Such a thoughtful man, that Negan.
After his wife's death, Negan joined up with another survivor in the hospital, but from what I can tell, the series is still ongoing. What comes next for Negan is unknown, and just how he bridged the gap from grieving husband to psychotic tyrant is uncertain. What has been shared so far proves that the more things change, the more they stay the same; Negan may have become more extreme, but the seeds of evil were there from the start.