In The Walking Dead #107, Rick Grimes travels to the Saviors' compound to rescue his son Carl, who stowed away on one of Negan's trucks. Rick fears the worst, especially once Negan hints he had Carl tortured, leading to a fight between the two. During the clash, Rick bites down hard on Negan's arm, with Walking Dead's creator confirming this moment was meant to send a message.
In Walking Dead Deluxe #107 from Robert Kirkman, Charlie Adlard and Dave McCaig, Kirkman notes that the bite was meant to send a clear message about how Rick differs from Negan. Kirkman writes:
"Did you f******* BITE me?!" That was a fun line of dialogue I knew I had to put in there. A moment where Negan is ignoring the pain of being bitten and is more indignant about the IDEA of someone biting him. I also liked how it made Rick Grimes seem so much more ruthless and less civilized than Negan. ... Anyway, good issue showing how sane Negan can seem, while totally being insane.
Negan Is More Civilized Than Rick... And That's a Bad Thing
Rick Represents a New Order, While Negan Is the Worst of Humanity
The entire issue stresses how reasonable Negan actually is. While the Saviors ruling over other communities is morally wrong, they have consistent rules and don't engage in abject cruelty for its own sake. Negan even states that killing Glenn was done to prevent more excessive bloodshed - Rick's group had to know that they were powerless to resist, otherwise they would have lost more people fighting back.
While this doesn't make Negan any less evil, it does emphasize that he's not as chaotic as he seems. Negan doesn't make decisions on a whim - he truly believes that his violent, fascistic world view is correct and that it's the only way humanity can survive the zombie apocalypse. Negan slaughters his victims and maims his own followers, believing it's the most effective way to keep order. In contrast, Rick is actually far more chaotic - he's governed by his own sense of morality and his need to protect his family and allies. Rick is prepared to do terrible things in service of those goals, and will tear down anyone who stands in the way of them.
Rick's First Bite Against a Human Turned Him into a 'Living Zombie'
But This Monstrous Moment Was a Vital Part of His Journey
In his commentary, Kirkman points out that Rick biting Negan calls back to Walking Dead #57. In that issue, a group of bandits attack Rick, Abraham and Carl. When the leader orders one of his men to rape Carl, Rick loses control, tearing the man's throat out with his teeth before beating his lackey to death, despite the fact that Abraham already has him at gunpoint. In the Deluxe reprint of that issue, Kirkman describes this as Rick becoming a zombie:
Rick essentially becomes a living zombie in order to save [Carl]. I'd had the "Rick bites a guy like a zombie" scene planned for a while. ... I just thought it would be cool to push things to the point where Rick had to behave like a zombie in order to survive. From there, it was just a matter of coming up with a situation dire enough to drive Rick to do that.
One of Walking Dead's most iconic moments is when Rick declares, "We are the Walking Dead," giving up on any hope of humanity regaining control. Later, he declares he was wrong, showing how his arc goes from traditional authority figure to hopeless 'living zombie' to the instigator of a better world. Rick makes society better than it ever was by enshrining new values, but before that, he had to be stripped of his prior perspective.
Rick behaving like a 'living zombie' - biting his enemies and doing everything he can to hurt them - shows him being stripped of his sense of 'civilization' in a way that ultimately leaves him open to better ideals. Negan can justify burning his followers with an iron because their obedience keeps his settlement going. However, having become a living zombie, Rick has firsthand experience of a dark lesson: humans are capable of anything, and the species should only survive if it actually deserves to.
Rick Grimes' Death Turns Him into an ACTUAL Zombie
Trying to Bite Carl Shows Rick Is Gone Forever
Of course, these instances of Rick becoming a 'living zombie' also foreshadow his eventual death in The Walking Dead #192, when he's assassinated by Pamela Milton's son. Rick is shot in bed, having just set society on the track to recovery in a better form. Carl later discovers him as a zombie and is forced to execute the shambling remains of his father before he can be bitten. Combined with Rick's earlier 'bites,' the moment makes it clear that he's truly gone, and that Walkers hold no essence of their former selves - Rick might have become a 'living zombie' to save Carl, but trying to bite his son shows there's nothing of Rick Grimes left.
Rick's character arc is perhaps the best part of the original Walking Dead comics, as he goes from small-town cop to mythic savior of humanity. However, the journey between those states includes a long plunge into darkness, with Rick's fight against Negan showing how far he's fallen. Giving up on the 'civilization' that Negan tries to embody ultimately helps turn Rick into the man Walking Dead's remaining humans need to lead them into a better tomorrow.