In the episode, we get a glimpse of his life, pre-apocalypse: he was an unemployed gym teacher, got fired for a violent outbreak, and was cheating on his wife, Lucille (Hilarie Burton-Morgan). But when she's diagnosed with cancer, he changed his behavior and doted on her as the apocalypse broke out. Though he wasn't fired for assaulting a man in the comics, it's at the start of the apocalypse that his TV-backstory takes a hard turn from the source material.
While the Walking Dead TV series shows Negan trying to keep a sick Lucille alive during the outbreak of the undead in the comics, she dies much earlier in the comic books. In the source material, Lucille is still in the hospital and dies at the very beginning of the outbreak. According to The Walking Dead Wiki, when she passes and becomes undead, Negan saves a teenage boy and asks him to put down Lucille. Then, Negan teams up with a string of people who last about as long as a redshirt on Star Trek. He tries to help the people he's with, but becomes jaded after so many of them die.
In contrast, in the series, Negan leaves Lucille alone to find chemotherapy drugs after their stash spoils. While trying to steal them, he's knocked out with a baseball bat and meets some doctors. After telling them about his wife, they kindly let him go. But then he's kidnapped by a gang that forces him to give up the medicine's location. When Negan returns to Lucille, he finds she killed herself and has turned into a walker. He burns down the house and then rescues the doctors, brutally killing the gang members. It's here that Negan realizes who he can be in the wake of the apocalypse — he doesn't have to abide by society's rules anymore.