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Sanity is a recurring theme in The Walking Dead. It is the ability to think and behave in a normal and rational manner. Many characters lose this ability during the zombie apocalypse. Typically, the loss of one's sanity is attributed to the death of a close family member, friend, or by a harrowing event.
Shane slowly becomes insane; facing the fact that he will never have Lori, that Rick now runs the camp, and that help isn't coming. He eventually has a breakdown and attempts to kill Rick. (Days Gone Bye)
Carol begins to lose her sanity after breaking up with Tyreese, first attempting suicide and then becoming increasingly clingy to Lori and Rick, going as far as proposing a polygamous marriage between the three of them. Her actions cause a rift between her and the rest of the group, particularly Lori and Rick, attributing to her slip into insanity even more due to isolation. This culminates in her committing suicide by letting a zombie bite her neck. (Issue 22-Issue 42)
The Governor becomes unstable after the death of his brother, becoming convinced that he needs to be as ruthless and cruel as he was if he wants to survive. He assumes his brother's identity, forcing himself to commit atrocities in order to harden himself. He also keeps the decapitated heads of his victims in aquariums and keeps his zombified niece in his apartment, even kissing her on the mouth at one point. (Novel Series, Comic Series)
Rick starts hallucinating after the death of his wife and baby. (Issue 48)
Morgan breaks down after the surprise death of his son, keeping the undead boy chained up in his house and feeding him at least four unnamed survivors. (Comic Series)
Ben kills his brother Billy, reasoning that he will "come back". (Issue 61)
Negan is shown to be slightly unstable as he goes from being cocky and playful to being ruthless and narcissistic in an instant. This seems to be the result of Negan being sociopathically detached from the horrific events around him after the death of his wife, allowing him to approach most situations with his trademark immature humor, no matter how serious or gruesome. (Something to Fear - Present)
Beta is extremely paranoid about his identity being revealed, frantically screaming once his mask was taken off. (Issue 173)
Negan breaks down in front of Maggie and begs her to shoot him in the head. (Issue 174)
Lilly becomes paranoid after supplies were vanishing around the motor inn and ends up shooting Doug/Carley in a heated argument when interrogating Ben on the side of the road. (Long Road Ahead)
The Stranger becomes psychotic after Lee's group steals his family's supplies from his station wagon at the end of Episode 2 because this led to the death of his family, keeping his wife's zombified head inside a bowling bag. (No Time Left)
It is possible that Nate grows psychotic after his friend is shot by Eddie. (Wyatt's Story)
Nate seems to lose his sanity when he reveres the idea of killing the old couple in the diner, going as far as killing them regardless of what Russell has to say. (Russell's Story)
Sarah loses her sanity after her father is devoured in front of her. She refuses to accept the fact that he is truly dead. (Amid The Ruins)
Minerva becomes deranged and delusional after being bitten. She leads a herd of walkers towards Clementine, AJ, Tenn, and Violet/Louis (Determinant). She confronts the survivors and wants her brother Tenn to join her in death. When the other survivors stop Tenn from joining her, she goes berserk and opens fire on them, leading a herd onto the bridge. This ultimately results in her demise, but also that of either her brother Tenn or, determinately, Violet or Louis. If Tenn is killed, Minerva dies with a smile on her face, since Tenn died with her like she had wanted. (Take Us Back)
After seeing Raymond Nalls' notes on a possible cure, mainly drawings of herself, Lilly Caul realizes that he's insane. As Nalls shows Lilly his bite wound and explains himself, she sees the extent of the madness crackling behind his eyes for the first time. (Search and Destroy)
Shane slowly loses his sanity after facing the fact that he will never have Lori and Carl for himself and that Rick now runs the camp, eventually leading to him attempting to kill Rick, threedifferenttimes. (Wildfire - Better Angels)
Hershel keeps a pocket watch as a reminder of his family.
Beth wants to commit suicide after the barn massacre. (18 Miles Out)
Hershel grows careless after the barn incident. (Nebraska)
Rick loses his sanity after learning of the death of Lori and begins to have hallucinations: having conversations with deceased people over the phone, seeing Shane as one of the Woodbury guards, and seeing hallucinations of Lori in her wedding dress and her previous clothing before her death. (Killer Within, Say the Word, Made to Suffer, The Suicide King, Home, This Sorrowful Life)
Carl thinks about naming Judith by using names of previously dead group members.
Morgan is seen to have lost his sanity after the death of his son at the hands of his undead wife, whom he finally had to put down. (Clear) Over a decade later, Morgan reveals that this whole period of his life is a blur with him only remembering bits and pieces of it. (King County)
Carl appeared to have lost much of his morals after the death of his mother. His thinking appeared to be radically different from that of a normal teenager after the final assault of The Governor's army at the prison. (Welcome to the Tombs)
Lizzie is shown to suffer from a form of mental illness and possess psychopathic tendencies. (Season 4)
Morgan slowly regains his sanity and a sense of purpose with the help of Eastman. (Here's Not Here)
Ron Anderson finds himself in a psychotic state when he tries to kill Rick after witnessing the deaths of his mother and brother, blaming him for the misfortune his family suffered by meeting him. (No Way Out)
Morgan suffers a mental breakdown after the loss of Benjamin and reverts back to his PTSD fueled insanity. (Bury Me Here)
Siddiq suffers from PTSD after witnessing his friends die at the hands of Alpha.
Beta scribbles "I am the end of the world" all over the walls of a hospital room and is extremely protective of his identity for unknown reasons. (We Are the End of the World)
Carol Peletier begins to hallucinate after the loss of Henry, likely as a result of sleep deprivation. (Ghosts)
Brandon is delusional and psychotic, with a perverse fascination with Negan and the Saviors. After Negan is freed, Brandon catches up with him, but instead of bringing him back, he joins the escaped convict. After Negan dismisses and insults him due to being repulsed by Brandon's personality, Brandon murders Amelia and her son Milo, in order to prove himself to Negan, by whom he is killed shortly after. (What It Always Is)
Virgil went insane when his actions led to the death of his family. He imprisoned his former colleagues and consumed copious amounts of hallucinogenic tea so he could see his family again. What We Become
After Negan kills Alpha, Beta experiences a full psychotic break and goes insane to the point of hallucinating the dead talking to him. (The Tower, A Certain Doom)
Martha goes insane after her husband Hank dies and starts to believe that helping people makes them weak, while not helping makes them strong, particularly if they turn into walkers. She thus begins murdering members of Clayton's group and targets Morgan's group. (Season 4)
After Martha poisons his friends, Morgan flies into a blind rage and nearly kills her as Martha wants, having tried to goad him into becoming "strong" again. Realizing what he's doing, Morgan stops and regains control of himself. (... I Lose Myself)
In the ruins of Albany, Hope Bennett comes into conflict with Candice Froder, a survivor who has been driven insane by the world around them. However, this ultimately proves to just be a hallucination caused by Hope's concussion. (Konsekans)
Arnaud is an insane young man who appeared to have lost his mind because of the horrors of the world, possibly related to witnessing the death of his father. Arnaud ends up becoming a serial killer murdering children, seeing it as a merciful act in order to spare them from the horrors of the world. (Davon)
Negan and Perlie find a disturbing effigy that Perlie suggests was made by someone who had gone insane after the world had ended. After finding the body of the survivor responsible for the effigy who had committed suicide, Negan realizes that the marshal was right. (Stories We Tell Ourselves)
The Dama is pleased to discover that Negan is actually real and not just a product of the Croat's insanity. (Doma Smo)
Major General Beale is revealed to be insane, plotting to wipe out all other survivors in order to scavenge their resources and ensure the survival of his community alone as humanity supposedly faces extinction in around 14 years. (The Last Time)