The main theme could be about redemption. In many such novels, characters who are in difficult or morally ambiguous situations strive to redeem themselves, like a 'dead man' in a sense of their past actions coming back to haunt them but still trying to make things right.
I'm not sure specifically as there could be many different versions of a novel with this title. But generally, there might be a protagonist who is in the situation of being a 'dead man walking', perhaps a wrongly accused man facing execution.
Yes, it can be. It can serve as an introduction to Negan's character, which is a significant part of the 'Walking Dead' story. Even new fans can quickly get into the story and understand his role.
The Walking Dead isn't classified as a visual novel. It's known for its intense zombie-themed storylines presented mainly through comics and TV. Visual novels have a specific format and gameplay elements that The Walking Dead doesn't possess.