AMC's The Walking Dead tends to run parallel with the graphic novel that inspired the story, occasionally checking in and out with the source material. At the beginning of Season 9, there are still a lot of storylines that the adaptation can cover from the original, the most important of which is when — or if — Negan gets out of jail in the Walking Dead series. On the show, he's currently locked up in the Alexandria-Safe Zone by Rick and Michonne for now. But fans of the Walking Dead comics know that that won't last forever.
In Issue One, Volume 22, "A New Beginning" in The Walking Dead comics, two years after all out war between the Saviors and Alexandria, Rick's teenage son Carl (who is still alive) goes to visit Negan in his basement cell. Just like the show, he is being kept alive after losing the battle. When a new group of survivors discover the former villain where he is being kept, he pretends to be an innocent victim and warns the new survivors that Alexandria is full of monsters with dark secrets. That ruse doesn't work, but Negan is at least still able to antagonize Rick and intrigue Carl from his cell from time to time.
When The Whisperers, a new villainous group, arrive and begin to terrorize the survivors, Rick asks Negan for leadership advice — but is still unwilling to free or kill his nemesis. More time passes. It isn't until Volume 26, "Call to Arms" that Negan escapes his cell. A character named Brandon Rose helps smuggle him out back to his home at the historical center. Brandon is a Hilltop resident with a grudge against Rick Grimes after learning that Rick killed his father.
Fun fact — Brandon's mother in the comics is named Tammy Rose, and a character by that name has already appeared on the series in Season 9. She's the woman whose son (on the show) died in the Smithsonian raid. She seems to hate Maggie, and probably Rick by proxy. Perhaps the pieces are being set for Negan's escape already.
After all, Negan does still have supporters within various communities on the television show. Anyone could make a play for him in Alexandria at any time. Could Negan ever be redeemed? There's probably at least one corner of the fandom that yearns for that, though most of the protagonists on the actual series would likely disagree.
"I think he's going insane," said actor Jeffrey Dean Morgan according to Screen Rant. "I think the four walls have closed in on him, so he's battling trying to keep some sense of sanity. And it's not going well. He's used to performing in front of a lot of people and putting on a show. Unable to do that, kill the party, is very important to his psyche. He's going through a lot of sh*t right now."
As far as the graphic novels go, however, he's supposed to stay behind bars for quite a bit of time. When Negan escapes in the comics, the Whisperer War is already on and he enters in the middle. To say anything further about Negan's role in that upcoming conflict would be a major potential spoiler. It gets bonkers, so let's leave it at that.