In the comics, Negan was a fucking psycho but he ruled the Saviors and did things the way he did because he thought it was for the greater good -- the twisted, completely fucked up and psychotic good. He genuinely felt that using murder and violence to keep people in line was saving far more lives in the long run. They illustrate him as being internally broken. Just a desensitized sociopath who suffered some kind of shut-down due to traumatic events, operating in the only way he knows how anymore. At the end of all the battling, Rick gives an impassioned speech about how Negan's way is wrong, and how everyone should work together. Negan actually agrees, says he's been wrong all this time, and that's when Rick makes his move.
But the show has made Negan such a silly, moustache-twirling cartoon character, that it'd be impossible to buy that scenario playing out the same way. In the comics, Negan seems to have a sort of moment of clarity when Rick tells him he's flat-out wrong for being such a tyrant. Negan has other moments like that - such as being genuinely sorry and apologizing when he hurts Carl's feelings over his lost eye - that foreshadow such a change of heart. But the show's portrayal of Negan makes him seem lucid and self-aware all the time, and just a complete asshole. Even when he's being sincere, he's still swaggering about, leaning back and aggrandizing himself. With the show, it's not an act or the behavior of an emotionally dead man -- it's the behavior of a man who's getting off on all of this.
So yeah, Negan needs to get offed. Not that the show has cared much about tonal consistency or careful writing, but the outcome from the comics can't happen in the TV show, with the way everything has been developed thus far.