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The room is small. Help is far away, on the other side of many locked doors. Your arm is chained to the table and a Rabid Cop is spraying spittle into your face in a way that convinces you that he has completely lost his mind.
All he wants you to do is admit that everything Hitler did was your idea. Sounds good to you. What do you have to sign to get away from this maniac?
The Rabid Cop might be casually dirty, or overbearingly self-righteous, or anywhere in between, but they all have two things in common: a reckless disregard for civil rights, and an unwavering conviction that any person they've identified as "the perp" really is a perp (regardless of any contradicting evidence) and deserves to suffer. In a Good Cop/Bad Cop routine, they usually take the "Bad Cop" ball and run clear out of the stadium with it. Likely to enjoy using Torture for Fun and Information.
Compare/contrast the (presumed) sympathetic Cowboy Cop.
Perhaps the ultimate example is Judge Dredd's nemesis, Judge Death. He starts off his time in his reality's judge force executing perps for minor crimes. And he only gets worse. Eventually, he reasons that all crime is committed by the living, therefore life is a crime. Cue the extermination of his world's entire population and his attempts to do this to Dredd's universe.
Bud White of L.A. Confidential is a heroic version, though he does frighten the officer playing 'bad cop' as well as the suspect at one time.
Detective Park Doo-man and Detective Cho Yong-koo from Memories Of Murder both brutally try to beat and torture confessions out of their suspects, one of whom was a mentally handicapped young man, and get very few results. They're contrasted with Detective Seo Tae-Yoon, who uses logic and reason in his investigation, but by the end of the movie, is driven to becoming almost as bad as them.
Henry Oak in Narc. In a similar vein to the aforementioned Alonzo Harris below, he happens to be a narcotic police officer with a case of Police Brutality against the criminals he's facing against.
Nick Tellis himself is not far off from being a violent cop too. The reason why he was kicked out of the police force was because of him shooting a drug dealer holding a child hostage, resulting in one of his bullets hitting a pregnant woman.
Saw: David Tapp in the first movie. His recklessness nearly kills one of Jigsaw's victims, gets his partner shot by multiple shotguns at once, gets his own throat cut, nearly killing him, and gets him dismissed from the force. And that is just backstory. By the time the events in the movie proper start, he is a broken shell of a man in a fetid bedsit across from the house of the guy he thinks is the killer. It is arguably creepier than any of Jigsaw's actual traps.
Detective Eric Matthews in the second movie. As the movie progresses, it's revealed that he has a very nasty record of violence towards suspects, and in several cases planted evidence to gain a conviction. And after spending most of the film watching his son trapped in a house with the victims of said evidence-planting, he resorts to thrashing the living daylights out of Jigsaw for the location of the house, which turns out to be a trap set up specifically for Matthews.
Although he isn't in the circumstance described above, Alonzo Harris from the film Training Day is the embodiment of this trope. He isn't insane though, just a sociopath.
Verbal Kint: The DA gave me immunity. Dave Kujan: Not from me. You get no immunity from me, you piece of shit!
Star Wars: It's the overriding characteristic of the Jedi Knights (especially Mace Windu) in the prequel trilogy. (The costumer even mentioned dressing the Jedi in black so they would look like intergalactic police officers.) They become increasingly vigilante-like and subversive in their attitude toward the Republican Senate, until they finally decide that they must execute (okay, forget "execute", lynch) Chancellor Palpatine in Revenge of the Sith because he is a Sith Lord and because he could just manipulate the courts into setting him free if he were ever arrested. During the attack on Palpatine, Windu seems to have totally lost his mind - and, in that light, Palpatine and Anakin Skywalker's murder of him looks less like the Dark Side and more like self-defense.
Well, for one thing, there's no way to deprive Palpatine of his Force powers other than rendering him unconscious, so there's a good chance he'd escape if they tried to detain him.
Live Action TV
Ray has moments like this in Breakout Kings, including threatening to burn a suspect's genitals with a cigarette lighter.
The aptly named Detective Slaughter from Castle, a one episode character played by Adam Baldwin.
Kate Beckett herself was this at various points, usually when the case involved her mother's death. After she is shot by a sniper she becomes this way when investigating that case as well as anything that reminds her of it. And when, in the first episode of season 7, she interrogates a low-level thug who she believes knows who kidnapped Castle - and she isn't sure whether he's still alive, but believes that he knows - she appears completely willing to break as many of the guy's fingers as is necessary to get him to talk. You do not get between Beckett and the people she loves.
Jimmy Beck in Cracker, once causing his superior officer to say 'I don't know what you did to him, but you scared the hell out of me.'
Before Andy Sipowicz, there was Mick Belker on Hill Street Blues. Dude even barked and growled like a rabid dog. Bit people on a regular basis. "You gonna tell me what I want to know or am I gonna have to show you my ass!"
Elliot Stabler from Law & Order: Special Victims Unit can be a violent person during his interrogations, especially when It's Personal. Which is roughly every other week. This tendency earned him the Fan Nickname Un-Stabler. Most likely because Elliot is increasingly unstable and psychotic. Many theorized the series would not end until he actually killed someone, ending his massive fall from grace... and ironically, while he did end up off the show due to a killing in the precinct it was A) a 'clean' (wholly justified) shoot of someone that was shooting up the holding cell and B) happened to be a 14 year old girl he was wholly sympathetic with.
The cop in the (ironically named) episode, "Unstable", made Elliot look normal.
Dana Lewis is also an example of this. Though she did have a running gag where, in every episode she was featured Elliot would end up being hurt in some way, the episode "Penetration" ends with her chasing down her rapist and cornering him in a warehouse with her gun drawn. This after being nothing but rude to Elliot and Olivia, and she actually tries to shoot the guy. The episode "Secrets Exhumed" ends up with her being arrested and thrown in prison for 25 years for killing her ex-boyfriend's girlfriend out of jealousy and spite.
Most of the 1973 detectives in Life On Mars are rabid by today's standards, especially in contrast to 2006 transplant Sam, but Gene Hunt deserves a special mention, here.
The retired detectives of New Tricks have slightly Cowboy Cop attitudes compared to modern police methods and standards. So they see nothing wrong with creating a fake Rabid Cop scenario where the interrogator gets so insanely angry that he shoots the suspect's public defender lawyer. The 'lawyer' is another retired cop and the gun is a starter pistol.
And occasionally they find themselves working alongside a real Rabid Cop, such as Frank Patterson in "The Fourth Man".
The Shield is about the Strike Team committing a lot of police misconduct in the name of stopping gangs and enriching themselves. Vic Mackey in particular counts as such. While he is a proponent of the Jack Bauer Interrogation Technique, Vic only rarely turns into the rabid cop - usually, his menacing is done with a cold and calculating air.
Jack Malone of Without a Trace can be pretty worked up and he will do anything to get information on on those poor missing people.
Several police on The Wire, but the standout examples are Anthony Colicchio, who attacks a teacher when said teacher asks him to move his police car, and Eddie Walker, who breaks the fingers of a preteen carjacker because the kid's joyride (and collisions with parked cars) gave him additional paperwork to fill out.
Discworld: THAT! IS! NOT! MY! COW!. Vimes does whatever he can not to turn into one, to the point that he actually has a Vimes-esque entity in his mind to prevent him from succumbing to the darkness.
"Who watches the watchman? I do."
Averted in The Dresden Files book Changes. Rudolph tries his best to play the Bad Cop, but all his desk-pounding and spittle-flecked screaming manages to do is cause Harry to crack up and the other interrogator ends up ordering him out of the room. It probably helps that Harry has seen Rudolph freak out whenever confronted with the sort of thing he deals with all the time.
Captain Zuccho from Incompetence has a Hair-Trigger Temper, to put it lightly. Asking him to calm down will result in him randomly shooting at the pavement. Reputedly, he is on Prozac, but it doesn't seem to be helping.
Agent Robert Nightingale in Alan Wake is a ruthless federal agent pursuing the main character. Though the source for his violent behavior is found in his back story.
Lt. Carter Blake in Heavy Rain is a psychopathic police officer with immunity from the local precinct (why, nobody knows) who prefers beating a suspect rather than extracting any information, has no problem with breaking the law in order to investigate, and will have no qualms about killing. Initially, he's rather reserved to just beating suspects, and then he roughs up a psychologist who has done absolutely nothing. And then does everything in his power to assure those affiliated with the investigation that Ethan Mars is the Origami Killer. The FBI agent attached to the investigation, Norman Jayden, isn't convinced, and the two have a very rough rivalry. If Ethan is arrested, then it leads to a scene where Blake will mercilessly beat Ethan into unconsciousness. Jayden can intervene and punch Blake, which will prompt him to hold Jayden at gunpoint, waiting for the perfect opportunity to kill him.
It doesn't stop there. One possibility at the end of the game has Blake ordering his squad of officers to gun down Ethan who had finally reunited with Shaun, his ten-year-old son, after having gone to incredible lengths to save him from drowning in the warehouse's well, all while the aforementioned son watches in horror as his beloved father's body falls to the ground, lifeless. All because Blake refused to believe anyone but Ethan could be the Origami Killer and the instant the obviously unarmed man clutches his left hip in agony instead of keeping his hands up, Blake gives the order to shoot. Needless to say, Blake is by far the most hated character in the game, even the actual killer doesn't come close. The worst part? In any good ending he gets away with everything.
Saren Arterius of Mass Effect 1. As a Spectre, he's essentially a Council space cop with no strings attached, and he plays it to the hilt - making frequent usage of the Jack Bauer Interrogation Technique, pursuing his own ambitions on the side and pinning the collateral damage on people he doesn't like. He's the same sadistic, racist government law officer we've seen in many other works - just relocated to a sci-fi setting. And this was what he was like before he went Jumping Off the Slippery Slope.
A good way to lose in Police Quest, acting like the aforementioned Carter Blake above will not be tolerated in the force.
A good number of Templars in Dragon Age are a little overzealous in attempting to contain the threat that mages pose. There are plenty of reasons to fear mages, but many Templars are willing to paint every mage with the worst brush and take the worst measures to make sure they don't stay a threat. By Dragon Age: Inquisition, most of the order becomes this when they go rogue from the Chantry in order to put down the mage rebellion by any means necessary.
The titular character from the [adult swim] show Assy McGee is an extremely violent parody of this trope (and a Heroic Comedic Sociopath) despite being, as his name suggests, a pair of ass cheeks. He was perpetually intoxicated, told to hand in his badge and gun at least once an episode, and other offenses too numerous to list here.
This is parodied in The Boondocks where a Rabid Cop violently accuses and assaults Butt Monkey Tom Debuoir for a crime that he obviously didn't commit before being forced out by the nice cop. He then rushed in 5 seconds later to assault Tom again.
Iron Man: Armored Adventures gives us their take on the NYCPD and the inexplicability of Family-Friendly Firearms at the same time. Doppelganger!Tony has just shot at unarmed people at a party with a laser gun and rushed off. The real Tony Stark is taken in for questioning, and one of the officers is like this, complete with banging on the table and yelling, "Did your friends give you the lasers?!"
The Springfield Police Department is sometimes depicted this way on The Simpsons. (To the tune of "Bad Boys" from COPS: "Whether in a car or on a horse, we don't mind using excessive force!")
Rancid Rabbit from CatDog. Saving the fact that he also serves as The Rival to Cat.