Contrary to many people's preconceptions that disabled people are all nice or otherwise pleasant to be around much like many depictions of the Magical Differently Abled Person, disability does not prevent the person with it from being an unpleasant person. Some will even use their "illness" to get away with saying things that are at best inappropriate, and at worst outright offensive.
Another variation will have the character claim to have the disability despite the lack of a professional diagnosis, or fake the symptoms so that they can get diagnosed with it. This character generally comes from a privileged background, and has thus become accustomed to treating other people poorly. Usually this character is just a Jerkass or Attention Whore, and would be exactly the same if they did not have or did not claim to have the disability. A popular subject for this over the Internet is Asperger's Syndrome
The "faking it" variation is a parody of a similar behavior in real life, which can be a Berserk Button for people who actually are disabled, due to the prejudice they are targeted with every day compounded with having to put up with ignorant assholes who are adding insult to their injury by claiming to have it just to get attention and/or have a convenient excuse for being a jerk.
A Sub-Trope of Acquired Situational Narcissism. When a disabled character is not entirely jerkass, but just snarky, they are a Disabled Snarker. When a disabled character isn't just a jerk but also evil, then it's Evil Cripple. When a character uses their age rather than an illness to get away with similar behavior its Screw Politeness Im A Senior.
There is some Truth in Television to it: a disability can have a serious impact on a person's personality traits. How much, depends on many factors: the circumstances that led to the disability, the type of life held before the accident (a bookworm may adapt to a wheelchair more easily than a sportsman), the level of support of friends and family, previous personality, etc. Of course, No Real Life Examples, Please!
In the 1990 Paul Hogan comedy Almost An Angel, the film's hero, Terry Dean, is in a bar when he encounters Steve Garner (Elias Koteas), a wheelchair bound Jerkass who is using his disability as an excuse to push other people around. Dean calls Garner out on his behavior with a veiled hint that if Garner doesn't stop being such a jerk, Dean will beat the crap out of him. The other bar patrons object, pointing out that you can't fight a guy in a wheelchair. So Terry Dean pulls up a chair, puts it within Garner's arms-reach, and sits down in it. He then raises his fists, saying that the fight will now be fair.
Frank from Scent of a Woman is the poster boy for this trope.
In House Rules, Jacob is a Jerkass and considers his autism as a legitimate excuse for being such, as well as that autism is the next step in evolution.
Averted with most of the disabled and/or Ill Girl characters in the Chalet School series, but one exception is Naomi Elton in Trials for the Chalet School. She is disabled as a result of a car accident, needing a cane to walk, and has what Elinor M Brent-Dyer calls a 'warped personality', using her disability as an excuse to be awkward and abrasive. Of course, she gets better.
The title character of Maurice Level's short story "The Cripple" lost the use of his hands in an accident with a grain-thresher, and receives regular compensation from a farmer who suspects him of faking the disability. On his way home after receiving a payment, he sees a girl drowning but is unable to grab her hand to save her. The end of the story reveals that he is indeed faking his disability, and let her drown to avoid giving himself away.
Connor of Degrassi. As soon as he was diagnosed with Asperger's Syndrome (never mind that the very portrayal of the disorder was totally inaccurate,) he decided that it was okay to be a complete dick to everyone, especially his friends.
The eponymous character in House justifies his jerkassery with his injured leg, but he was a jerkass before that. In one episode, Wilson argues against diagnosing House with Asperger Syndrome because he'd just use it as an excuse to be an even bigger jerk (though his jerkassery often brings some humorous moments).
In an American episode of The Office, Michael suffers from a very mild burn on his foot, and ends up using a wheelchair.
Sugar Motta from Glee uses her (self-diagnosed, this is important) Asperger's Syndrome as an excuse to be rude to everyone.
A combination of Sue Sylvester's protection and this trope keeps Downs Syndrome sufferer Becky Jackson from ever having to answer for her actions. Sue encourages her to take advantage of this.
Bones averts this completely with Doctor Temperance Brennan's clear case of Asperger's Syndrome. She has little grasp of social cues, general etiquette and people's emotions, but if anyone points out that her actions or her words may have caused offense, she is immediately apologetic. At first, Booth doesn't seem to understand that she genuinely doesn't understand some of the basics of social interaction. However, her best friend Angela is always ready to interpret for her and her colleagues are aware of the problems she has. In later seasons, Booth begins to be able to translate between herself and strangers like the local law enforcement. She does seem to be getting better at it as she spends more time interacting with strangers and talking to a therapist.
Her cousin, showing signs of Asperger's Syndrome that are as extreme as Brennan in the early seasons, also quickly tones down the behavior in question (citing quotes from Benjamin Franklin in response to anything and everything) as soon as somebody mentions it being an issue.
Paul and his wife were once harassed by a man in a wheelchair in an episode of Mad About You. Once Paul realized the man was disabled, he backed down, only to receive a blow to the gut. On instinct, Paul punched the man in the eye, resulting in everyone yelling at him for attacking a disabled person while the man in the wheelchair exaggerated the amount of pain he was in.
Dr. Romano of ER was always a jerk, but claimed he grabbed various nurses due to a malfunction of his prosthetic arm.
Roy (Chris O'Dowd) in The IT Crowd episode "The Work Outing" pretends to be disabled in order to cover a series of lies stemming from getting caught using the handicapped restroom at a theatrical performance. Each of his lies, however, only serves to make the situation worse for him.
In Night Court, a recurring character, a blind woman, was bitter, irascible and mean to everyone. Because of her condition, few people call her out on this.
Ironically before she lost her sight she was an even bigger jackass.
Elliot's brother Donnie faked a mental disability for twenty years just so that he could get away with mooching off the family and not have to work.
In another episode, Maya dates a blind guy who turns out to be a jerk to everyone else. She continues dating him despite this, until he asks her to describe porn for him.
The Lou and Andy sketches in Little Britain were built around the concept of a jerkass who is pretending to be wheelchair bound and possibly having learning disabilities in order to both be lazy to the nth degree and get away with being demanding ("Want that one..."), contrary ("...don't like it."), and to get away with some pretty horrible and ludicrous behavior up to and including murdering a woman who wouldn't take his attitude. All this to the complete unawareness of his carer Lou Todd, a rather sweet put upon man who Andy basically treats as a slave.
Facejacker: Dufrais, one of the personas, lives off this trope. He is in a wheelchair, but is an unpleasant, mean-spirited Jerk Ass.
Kids in the Hall featured Bruce McCulloch in a recurring role as an unpleasant guy who tries to elicit pity sex from women because he has a cabbage for a head (seen as cabbage leaves in place of hair).
SCTV station manager Guy Caballero is always seen in a wheelchair, though he doesn't need it; he just uses it "to get respect."
Saturday Night Live played a "lost ending" to It's a Wonderful Life where the townsfolk formed a mob to wreak revenge on Mr. Potter - when they find out he doesn't really need his wheelchair they get furious and really lay into him.
One recurring sketch near the end of MADtv featured a Jerkass mentally man who constantly insulted everyone around him, but always got away with it due to his disability.
Ana Guerrico, from Padre Coraje. Falling in love with the priest, and being told that she can't do that, does not help much...
In The Inbetweeners, Alistair was a Jerk Ass before his kidney transplant left him in a wheelchair and his illness didn't humble him at all. It's made funnier because the people around them see him as Inspirationally Disadvantaged, which he exploits to get attention from girls.
Jay: "He's a dick! I never liked him when he was well, I never liked him when he was ill and don't like him now he's getting better... He was a complete bell-end. He was worse than Briefcase... He's not going to have got more interesting in a hospital bed attached to a drip for a year... and he used to stink."
It's far from being the only excuse he gives, but Rimmer in Red Dwarf sometimes blames at least some of his awfulness on the fact that he suffers from the worst disability there is: he's dead.
From the day he came out of his coma, numerous General Hospital characters have used Jason Morgan's brain damage as an excuse for his horrible behavior. To the point where his new girlfriend Robin instantly forgave him for cheating on her with Carly because he supposedly was genuinely unable to understand that it was wrong to sleep with someone else while dating her.
Something Positive: As a young boy, Fred was admitted to a hospital for an ear infection, where he was antagonized at every turn by a boy in a wheel-chair, who used his disability as an excuse to rage at the world, and every other child at the hospital hates him. Eventually, Fred and his friends team up to scare him and teach him a lesson... whereupon it is revealed that the boy is in the hospital for a heart condition, and the shock of the scare ends up killing him.
Mituna, Sollux's Dancestor in Homestuck, tends to burst out into profanity and Anti-Seadweller Slurs frequently. However, this is because he suffers from severe brain damage that has left him with a split personality and intellectual disability, compounded by several speech impediments and a particularly frustrating typing quirk. As a result, most people don't take his outburst too seriously.
Terezi's blindness has never stopped her, but she does use her disability as an excuse to troll people for inadvertently using vision-based metaphors.
In a later episode, Chris goes on a date with a girl with Down Syndrome, who ends up being incredibly abusive to him. Finally fed up with the abuse, Chris calls her out on her behavior, but she refuses to change her ways and breaks up with him.
Inverted with Joe, who is paralyzed from the waist down, but is generally outgoing and friendly. In one episode where Joe regains the ability to walk, NOT being disabled makes Joe shun his friends and his wife in favor of a set of new friends. It isn't until Joe gets crippled again that he is brought back down to humble levels.
In The Proud Family, Penny dates a disabled boy, Johnny, who seems angelic at first, but turns out to be a rude prick who uses his disability to get his way.
In South Park, Eric Cartman once claimed to have Tourettes so that he could get away with making racial slurs and insulting people. This is only assisted by the self righteousness of a Tourettes' spokesman, who interprets every suspicion about Cartman as malicious bullying. He's eventually Hoist by His Own Petard, though, as his ability to self-censor disappears, meaning his outbursts reveal some very embarrassing things about himself.
In the episode "Ass Burgers" he attempted to do this with Asperger's Syndrome, but failed due to a very basic misunderstanding of the condition. (He thought it meant that burgers come out of his ass.) The episode itself also claimed that autism doesn't exist and all people who suffer from it are always jerks.