A person suffering from a long-term or terminal illness is under considerable stress both physically and psychologically, and this can adversely affect them in numerous ways beyond what their condition is doing to them. In some cases, their pain and degradation can cause a markedly negative change in their personality. They may become extremely short-tempered, foul-mouthed, and/or spiteful, venting on whoever is in earshot, sometimes to the point that it's intolerable to be around them. They may also do physical things such as throwing or breaking whatever's nearby, fouling their clothes and/or bed, or even trying to physically injure people. This abuse may get to a point that their caretaker(s) decide that abandoning them, locking them away, or even going as far as a Mercy Kill or just straight-up murdering them, is the only way to get some peace back in their lives.
People suffering from Alzheimer's disease or similar dementias can be just as susceptible to this, as the deterioration of their brains can lead to behavioral changes that manifest as hateful words and actions. Injuries can also lead to such a change, especially if they're crippling, cause chronic pain, rob a person of one or more of their senses, or affect their brain or mental state.
Compare the Annoying Patient, who's needy, whiny, and demanding to the people taking care of them but isn't necessarily hateful or abusive.
Contrast Disability as an Excuse for Jerkassery, when a person uses their sickness or infirmity as a convenient license to be nasty to people.
- The Boys: Downplayed with Tek-Knight (a Composite Character of Iron Man and Batman), who was found during his autopsy to have a fist-sized brain tumor. This caused him to have near-uncontrollable urges to have sex with almost anything he saw (not even limited to people either; his therapist kicks him out after discovering he'd had sex with the therapist's cup of coffee), although he was more or less able to restrain himself in public. He even came uncomfortably close to assaulting his Kid Sidekick Laddio.
- Green Manor: MacMunay is one of the regulars at the club where Extreme Doormat George works as a waiter and is constantly abusive towards him, even threatening to have him fired. However, George doesn't hold it against MacMunay, knowing his irritability is due to an old war wound. When Lord Virgil forces George to choose a man to be killed by Lord Virgil's assassin, he uses disappearing ink to write MacMunay's name and reappearing ink to write Lord Virgil instead.
- Me Before You: The protagonist spends much of the first few minutes being a jerk to his caretaker Louisa, bitter about being left a quadriplegic following an accident. When his ex-girlfriend visits, she tells Louisa "I tried for months. He just pushed me away, didn't want me here", revealing that he was one to her too.
- Armadale: Mrs Milroy's chronic illness brought her vicious, jealous qualities to the fore.
Suffering can, and does, develop the latent evil that there is in humanity, as well as the latent good. The good that was in Mrs. Milroy’s nature shrank up, under that subtly deteriorating influence in which the evil grew and flourished. Month by month, as she became the weaker woman physically, she became the worse woman morally. All that was mean, cruel, and false in her expanded in steady proportion to the contraction of all that had once been generous, gentle, and true.
- Dolores Claiborne: During the last years of Vera's life, she devolves into a helpless bed-ridden old woman frequently hallucinating about "dust bunnies". Her only source of joy ends up being intentionally shitting her bed in order to make her caretaker, Dolores, clean it up.
- The Empirium Trilogy: The bedridden King of Borsvall treated his son, Ilmaire, horribly, calling him weak and unworthy of the throne. However, considering he was possessed by an angel at the time, its ambiguous how much of his verbal abuse was him and how much of it was from the angel.
- Discussed in The Fault in Our Stars. Augustus' former girlfriend Caroline had brain cancer, and what he calls the Asshole Tumor. Lacking any sort of filter between her thoughts and her mouth, she'd often be rude and even downright mean to everyone, including Augustus. He admits that because he only met her after she got sick, he can't actually be sure if her behavior was due to the tumor, general pain and bitterness, or just her personality.
- The Green Mile: Melinda Moores, the wife of Cold Mountain Penitentiary's warden, develops an inoperable brain tumor. Her condition starts out as severe headaches, and as she declines, she begins to lapse into swearing. It initially sounds like Hollywood Tourette's, albeit Played for Drama as it's so obviously unlike her, but becomes so crude and malicious that it's described like Demonic Possession.
- Horns: Merrin Williams' older sister developed an aggressive type of breast cancer, and wasted away to almost nothing as the disease ravaged her body. Her father describes her to Merrin's boyfriend Ig Parrish as "barely eighty pounds, and seventy of that was hate." She viciously verbally abused both Merrin and him, hating Merrin for keeping her beauty and her intact body while she lost her hair and breasts and saying that her father loved Merrin more than her and was glad it was her that was dying.
- Nowhere Stars: Dowplayed by the main character. Liadain isn't cruel, but she is bitter, cynical, and snarky. Being diagnosed with a terminal immune disease at thirteen and knowing you probably won't live to see fourteen will do that to a girl, but even then she manages to at least be polite and is rarely ever openly rude or insulting to people who haven't done anything to her. However, it's also why she's willing to engage in some morally dubious actions, like using her Life Drain magic on sleeping civilians (something Keepers are really not supposed to do) to fend off her worst symptoms. This doesn't help her reputation as a Dark Magical Girl.
- Pet Sematary: Rachel's older sister developed spinal meningitis. She lived in constant agony as the disease twisted her body, and as her medicine became less and less effective, she began to take it out on Rachel and her parents. She would deliberately piss in her bed and frighten Rachel by touching her when she wasn't looking with her deformed hands. By the time she died, her drugs were completely ineffective and she spent all her time screaming because of the pain, which traumatized Rachel even further.
- Sherlock Holmes: The culprit in "The Sussex Vampire" turned out to be not Robert's second wife (who'd been caught sucking blood out of her child's neck) but his son from his first marriage, who had a deformed spine and an unhealthy attachment to his father. This, combined with his hatred for his perfectly healthy newborn stepbrother, made him stab the baby in the neck with a curare-tipped dart.
Holmes: It is a distorted love, a maniacal exaggerated love for you, and possibly for his dead mother, which has prompted his action. His very soul is consumed with hatred for this splendid child, whose health and beauty are a contrast to his own weakness.
- In Wuthering Heights, Mr. Earnshaw was always a stern parent who favored the foundling Heathcliff over his biological children Hindley and Cathy. But during his final illness he becomes far more irritable, intolerant of any rebellion against his authority, and especially verbally abusive to the wild, young Cathy. This is made even worse by the fact that the weaker he becomes, the more his religious bigot servant Joseph is able to influence him and encourage him to "rule his children rigidly."
- All My Children: After being paralyzed in an accident, Adam Chandler became quite rude and cruel to nearly everyone around him, including his much younger wife Gloria, though this was partly due to his genuine fears that he'd never be able to function sexually again. Despite Gloria's repeated efforts to assure him of her love, his relentless hostility resulted in his worst fears coming true and did indeed finally drive her into the arms of another man.
- Battlestar Galactica (2003): Invoked in "Escape Velocity" when President Roslin is dying of cancer while Gaius Baltar and his cult of followers are stirring up trouble.
Roslin: I'm going to be slipping away from this life very soon. And I've gotten kind of curious as to what that's going to be like, and so, I did some research. And there are some people who say that when people are getting closer to their death, they just don't care as much about rules and laws and conventional morality.
Baltar: Are you threatening me?
Roslin: No, no, I'm just saying have a quiet life, and I'll die a quiet little death. And everyone will be happy. It's just that I'm not in the mood any longer to indulge you.
- Coronation Street: After getting into a car accident that caused brain damage, Nick would occasionally fly off the handle and go into rages, including one Christmas where he slapped his then-wife, Leanne, in the face because her loud singing was aggravating a migraine he had and his requests for her to stop went ignored by her.
- General Hospital: Jason Quartermaine awakened from a coma after an accident with a personality that was a complete 180° from who he'd been before. He became quite rude and cruel to nearly everyone in his life, particularly his girlfriend Keesha, and suffered from amnesia that left him unable to remember anyone either. Even though his attitude and anger management eventually improved slightly, he remained mostly devoid of emotion.
- The Golden Girls: Downplayed for comedy with Sophia. She's a cantankerous (albeit often hilarious) old lady who says exactly what she thinks at all times. It's explained more than once that she had a stroke sometime prior to the series beginning, and that the stroke effectively destroyed her mental filter, so she can't help voicing whatever opinion pops into her head.
- The Haunting of Bly Manor: In the backstory, Viola Willoughby became sick with tuberculosis but managed to cling onto life for years, with her husband and her sister nursing her. Unfortunately, her debilitating illness—especially as she'd been fiercely independent before—and growing feelings of isolation from her family made Viola increasingly bitter and resentful. She became especially paranoid that her sister Perdita was trying to steal her husband and replace her as a mother figure to her daughter, causing her to lash out verbally and physically at Perdita, though she always forgave her. Perdita eventually snapped and fatally smothered Viola, resulting in her becoming a Vengeful Ghost.
- I, Claudius: Exaggerated by Caligula. He was already an unpleasant person prior to becoming Emperor, but shortly after taking the throne, he came down with a very serious illness, possibly malarial meningitis. He recovered, but had descended into madness, suffering delusions of godhood and keeping everyone around him in fear for their lives due to his insane behavior and capriciously murderous whims. He caused so much chaos that he was only Emperor for 4 years before being assassinated by his own bodyguards.
- Interview with the Vampire (2022): It's implied partly why Daniel Molloy is so brusque and grouchy is due to his Parkinson's disease taking a toll on him. The fact that he's dying also seems to free him to be direct, prying, and extremely critical of his interviewee Louis de Pointe du Lac — after all, what's Louis gonna do, kill him?
- Without a Trace: One of the two missing people in "In The Dark" is Kelly Corcoran, a teenage girl who recently went blind. Her introductory scene and one flashback depict her as being very bitter about her situation and especially prone to lashing out at her parents.
- Warhammer: Inverted in both this game and its futuristic spinoff Warhammer 40,000 by worshipers of the Chaos god Nurgle. They are ravaged with diseases and pestilence, but they view their afflictions as gifts from Nurgle that show his favor to them, and those that aren't insensate zombies are usually jovial and friendly and want nothing more than to share Nurgle's love to everyone around them...which just ends up spreading the diseases they're infected with.
- The Forgotten City: Rufius suffers from painful rheumatism which makes him surly and aggressive. Notably, when he's given proper treatment, he becomes a lot more friendly, and even apologises for his previous behaviour.
- Red Dead Redemption II: There are numerous cataclysms for Dutch's descent into madness and becoming his own undoing of his gang (including the loss of Lenny and Hosea, and the implication that his noble personality was an act all along), but his concussion in Saint Denis during chapter 4 definitely had mental consequences, as he becomes noticeably more angry, impulsive and merciless shortly after that point.
- Silent Hill 2: Mary Shepherd-Sunderland, James' wife, contracted a terminal illness which left her bedridden and caused hair loss and damage to her skin. She suffered from constant pain, frequent mood swings and suicidal depression, and would lash out in anger at James. He eventually smothered her with a pillow, although he repressed the memory of doing so and has delusions that she's still alive which lead him to Silent Hill.
- Trauma Center: Veronica Cage was infected with the Rosalia virus, which caused her excruciating pain and made her emotionally unstable. She eventually snapped, turning violent and repeatedly assaulting her parents. Terrified by the increasing severity of her outbursts, her parents locked her in her bedroom rather than seeking medical help, leaving her to die screaming for them while vomiting blood and clawing at the door until her fingers bled.
- Exvulnerum: The titular curse tortures its victim every night, leaving them not just in chronic pain, but also sleep-deprived and under constant stress. No matter how nice they were before, the victims end up lashing out at everyone, telling horrible lies, and doing whatever it takes to minimize the pain. Oh, and the only way to make it stop is to give the curse, and all its tortures, to the person they love the most. Despite how cruel this option is, pretty much everyone chooses it within a matter of months.
- Stand Still, Stay Silent: After a victim is infected with the Rash, they eventually start to become aggressive and are unable to recognize loved ones. On occasion, people close to the patient have attempted to break through to them, though they're always met with failure. Tuuri chooses to commit suicide after her infection is confirmed, knowing she and everyone around her will only suffer otherwise.