Cady starts off infiltrating a Girl Posse to get revenge on the Alpha Bitch, Regina, but over the course of the movie, eventually becomes a cruel, mean-spirited alpha herself.
Janis qualifies just as much, and even caused Cady's example to start with.
Mean Girls 2 also has this. Tyler points out in one scene that Jo is acting a lot like Mandi.
John Tucker Must Die is similar to the above: John has three girlfriends going at once and pretends to individually love each of them. They find out about each other, and concoct a plan to humiliate him which involves a fourth girl pretending to be in love with him.
James Cameron's The Abyss is a literal and figurative interpretation of this. The movie opens with part of the Nietzsche quotenote the home video version did, not the theatrical release, thanks to another movie using it that year, and the plot revolves around civilians and SEALs looking into an actual abyss & finding intelligent life. As they analyze the water creatures and debate over whether to destroy them with a nuke, the creatures silently watch humans and civilization on the surface above. In the longer cut version on the special edition DVD, the creatures very briefly threaten all of humanity with destruction, after having studied it for some time and realizing they might be in danger themselves. When you consider that current evolutionary theory states all carbon-based life probably originated in the oceans, this trope takes on multiple layers of meaning with regards to survival and destruction.
All the King's Men in a nutshell: Willie Stark counters corrupt politicians but becomes one in the process.
Knockout Ned from City of God slowly turns into this after his girlfriend was raped and his house was shot up.
Two-Face: You either die a hero, or live long enough to see yourself become the villain.
Batman Forever has one almost as good when Bruce lectured Dick about what would happen if he killed Two-Face.
"You make the kill. But your pain doesn't die with Harvey, it grows. So you run out into the night to find another face, and another, and another... until one terrible morning, you wake up and realize that revenge has become your whole life. And you won't know why."
Kevin Bacon's character from the 2007 film Death Sentence. Lampshaded by the gang leader in the ending, who, in a Not So Different moment, revels at the fact that he has reduced the protagonist to such a pathetic shell of a man.
Sheriff John Quincy Wydell in The Devil's Rejects is this trope to a 'T', as his quest to get revenge on the Firefly family for the death of his brother turns him into a sadistic maniac just like them.
In Fail Safe, Professor Groeteschele is arguing with Air Force General Black over the merits of launching a first-strike nuclear attack against the Soviet Union in the wake of a technical malfunction that sent a U.S. bomber to drop a bomb on Moscow, arguing that the threat posed by Communism justifies it.
Groeteschele: How long would the Nazis have kept it up, General, if every Jew they came after had met them with a gun in his hand? But I learned from them, General Black. Oh, I learned.
Black: You learned too well, Professor. You learned so well that now there's no difference between you and what you want to kill.
The book it was based on adds in some background to this: Groeteschele's family were German Jews (IIRC), and his father saw what was coming with the Nazis and emigrated out of there quickly. Said father often argued that point with his fellow Jews in America, claiming that if enough German deaths racked up trying to exterminate the Jews, they might rethink their policy. So it's a little of this trope and Well-Intentioned Extremist.
Tyler Durden of Fight Club. By the end, he's shaped his group to be just as conformist as the consumerist society he's trying to overthrow, and in some cases, it's even worse.
F.I.S.T.: over the course of the film, the title labor union begins to act more and more like the Corrupt Corporate Executives they were striking against in the beginning. This is driven home in the scene where they break up a wildcat strike (a strike unauthorized by the rest of the union) by force.
The Godfather: Happens to both Vito Corleone and his son Michael.
In his backstory, Vito ran away from a local mob boss in Sicily, only to become one in the process by killing a rival and taking over his turf.
Michael originally joined his family to protect their turf from the rival mob bosses, but becomes just as ruthless and cold-blooded as them over time. But by the end of The Godfather Part III, he's come to regret it.
Koba in Dawn of the Planet of the Apes. When the apes believed that all the humans were dead or gone, Koba had a Big Damn Heroes moment at the moment at the beginning of the film and convinced Ceasar's son Blue Eyes that "Scars make you stronger".
By Halloween 5: The Revenge of Michael Myers, Dr. Loomis is ready to use blackmail, threats, and physical force to make sure Michael Myers is put down for good. It goes so far that he uses Jamie, a 9 year old girl, as bait to lure Michael into a trap, and then beats him savagely with a plank of wood until Michael was unconscious. And yet he continues to beat him, all while screaming "DIE! DIE!" for each hit.
Being a Professional Killer who considers murder as part of his "job", James Bond at times doubts the morality of what he does and fears the potential of becoming just as evil and monstrous as his enemies. But despite taking a grim view, he refuses to let that shake his incredible loyalty to Her Majesty's Secret Service and country.
It does happen to him in Licence to Kill. It starts with Colombian drug lord Franz Sanchez feeding James Bond's longtime friend and ally Felix Leiter to a shark (after raping and killing his new bride), followed by Bond resigning from MI-6, briefly going rogue, and killing every member of Sanchez's organization in increasingly graphic ways. For once, Bond is not following orders, and is acting out of a thirst for revenge.
The Anti-Villain from Law Abiding Citizen. In fact, it seems like illustrating this point was his entire plan (beyond the immediate revenge on the men who raped and murdered his family). Just look at his satisfied reaction to be turned down for a plea or any sort of deal at the end. He wanted to shake the system so hard, the broken bits would be exposed, showing everyone who would look the kind of monsters that people become and allow when they start making compromises on Justice. Also, he was nutso-bonkers.
Yuri Orlov in Lord of War remarks upon this in reference to the revolutions in Africa:
Yuri: "I guess they can't own up to what they usually are: a federation of worse oppressors than the last bunch of oppressors. Often, the most barbaric atrocities occur when both combatants proclaim themselves freedom-fighters."
Will Graham in Manhunter and Red Dragon is a criminal profiler who lives in fear that his understanding of the mind of a killer will turn him into a sociopath.
Steven Spielberg's Munich controversially depicts "Operation Wrath of God", the covert assassination operation conducted by the Israeli government in retaliation for the murder of 11 Israeli athletes at the 1972 Summer Olympics in Munich, as being an example of this.
The Purge: When a gang of psychopathic killers invades your home and intends to kill you, it's a good question of how far will you go to protect yourself.
Played with in the movie Ravenous (1999). The villain gained power through cannibalism and the only way the protagonist can fight him is by partaking in cannibalism himself.
In Red Dawn, Tanner, a professional military officer, is concerned about this regarding the kids:
Tanner: "All that hate's going to burn you up, kid."
Robert:(carving 'kill' notches on his AK-47 with a balisong) "Keeps me warm."
Red Hill has this as an important part of the plot. Jimmy Conway's pregnant wife is murdered and raped, and he is set on fire. He doesn't die, but spends 15 years in prison after being framed for his wife's murder before he escapes and systematically hunts down and kills the men responsible.
Van Zan from Reign of Fire will do anything necessary to bring down the male dragon, including press-ganging members of Quinn's homestead when not enough of them volunteer.
Referenced by the Free French member Private Leroux — aptly nicknamed "Frenchie" — of the Rag Tag Bunch Of Misfits in the 1995 remake of the movie Sahara. He was sent by La Résistance to Africa to fight as a regular soldier since he had "too much hate [for the Germans] to make good Resistance."
Serenity: The Operative is fully aware that in his quest to make the Alliance safe for its citizens he's fallen into this, and states that on his list of monsters, he's right there at the bottom of the list and will kill himself when he's done.
Star Trek: First Contact: Picard is so blinded by rage and vengeance that he becomes as cold and unsympathetic as the enemy he's fighting, to the point of alienating (heh) his officers and refusing to consider the plan that represents their best chance of success.
By the time of Revenge of the Sith, in fact, the Jedi are becoming concerned that the whole Order is falling victim to this trope. They've been fighting in the Clone Wars non-stop for three years, a war of dubious morality that relies on slave labour (using mind-controlled soldiers who are raised from birth to do nothing but fight), the Republic has become increasingly more like a dictatorship with power concentrated in the Chancellor's hands, and things have gotten to the point that the Jedi are considering attempting a coup against Chancellor Palpatine in case he refuses to leave office after the war is over. Grand Master Yoda in particular has grave doubts about where the war will lead them even if they do defeat the Separatists and find the Sith Lord behind all of their woes.
The eponymous character of Sweeney Todd: The Demon Barber of Fleet Street, who starts the movie only wanting revenge upon Judge Turpin for sending him away to Australia and raping his wife, starts committing the murders that would mark him as the Demon Barber of Fleet Street after "Epiphany" and "A Little Priest", finally going off the deep end at the climax of the movie when, after finally exacting bloody vengeance upon Judge Turpin, he tries to murder his own unrecognized daughter, Johanna Barker, and then, after Mrs. Lovett screams down below and unwittingly saves her, he investigates the scream and finds out that the beggar woman that he killed just prior to killing Turpin was actually Lucy, the wife that Mrs. Lovett told him had died from the poison she took because she wanted him for herself. Sweeney, in a truly dark rage, throws Mrs. Lovett in her own oven to be burned alive. Then he settles down next to Lucy's lifeless body, where he gets killed by little Tobias Ragg, who saw the whole scene in the bake-room from the sewers after having to endure a seriousordeal to boot.
Thirteen Days. Dean Acheson, one of Kennedy's advisors, saying that the country's been "at war with the Russkies for twenty years", advises Kennedy to deal with the Cuban Missile Crisis, essentially, by invading Cuba and hoping that Russia doesn't have the nerve to let the retaliation lead up to nuclear war. His justification was that "the only thing [the Russians] understand is force." In real life, Acheson was more moderate, with this reasoning more in line with colleague Curtis LeMay.
This is one of the most important themes in Park Chan-Wook's Vengeance trilogy.
In Sympathy for Mr. Vengeance, the father sees his daughter on a coroner's slab and nearly faints. Later, after he's resolved to revenge, he watches the autopsy of another child and yawns.
Most obvious in Oldboy (2003). Both Oh Dae-su and his target Lee Woo-jin are aware of the trope. Oh plainly states "Now I have become a monster." Lee utters the meaningful line "Farewell, Oh Dae-Su," and refers to him as "Mr. Monster" from then on.
In Sympathy for Lady Vengeance, it is a crucial element of the plot. The movie doesn't end with the main character's revenge, but with her realisation of what she has become, and her vow to finally clear herself of sin.
"...the paradox in Magneto's character is that he was the victim and then becomes the aggressor. It's like he's slowly become these people who persecuted him and murdered his family right in front of him. He became embittered. You get angry enough and you start forgetting."
X-Men: First Class: Erik Lehnsherr hates Sebastian Shaw and wants to kill him, but he eventually embraces Shaw's beliefs about mutant supremacy. It's even spelled out through the villain wearing the same helmet that Magneto is associated with. At the crucial moment, he separates revenge from his ideals, which is why he's able to compliment Shaw's vision while still hating the man to his core. Shaw-the-man wronged him terribly, but Shaw-the-visionary is inspirational.
This seems to be the overall theme of Star Trek Into Darkness, with the "monsters" being both Khan and the looming threat of war with the Klingons. Both Kirk and Spock come dangerously close to becoming much like that which they were fighting. Kirk's intended vengeance against Khan for killing Pike before Spock talked him out of it, and Spock's intended vengeance against Khan until Uhura talks him out of it. Admiral Marcus, in his endeavor to steel the Federation against the probability of war with the Klingons, ends up becoming a warmonger himself. In the epilogue, Kirk even acknowledges it during his eulogy at the funeral:
Kirk: There will always be those who mean to do us harm. To stop them, we risk awakening the same evil within ourselves. Our first instinct is to seek revenge when those we love are taken from us...
This is basically Matias's character arc in The Elite Squad. He originally believed in justice, but the more he learns about the corruption of the normal police and the depravity of the drug dealers, the more he dies inside. After some losses that hit close to home, he snaps. By the end of the film he's fully embraced BOPE's brutal ways.
In The Star Chamber Hardin starts to feel they're becoming this when the secret court won't stop the killing of two innocent men in order to protect themselves and because they rationalize them as likely deserving it anyway.
Aberline spends most of The Wolfman (2010) bent on killing Lawrence, but in the end he's bitten by Lawrence and the curse is passed on to him.
Highwaymen: When Cray saves Molly from Fargo (a serial killer who uses his car as a weapon) the first time, Fargo offers another face-to-face meeting in exchange for the girl. When Cray takes him up on the offer, Fargo laughs and notes that Cray is becoming more like him the longer he's pursuing him.
Near the start of Viva Zapata! (1952), Zapata (Marlon Brando) has an audience with dictator Diaz, and is deemed to be a potential threat. At the end of the film, Zapata himself is the autocrat, and is challenged by a young firebrand in a scene which mirrors the earlier one.
Subverted in The Last Witch Hunter. Kaulder has been fighting witches for over eight centuries and many of them believe him to be a fine example of this trope in action, but as we get to see more of him, he turns out to be more of a Nice Guy.
"You should be careful how you fight evil. Your weapons can turn back on you."
Explored and discussed in I Sawthe Devil by Kim Ji-Woon. In the film, Kim Soo-hyun, a government agent, has sworn to violently avenge the murder of his fiancée, who was murdered by Jang Kyung-chul, a psychopathic serial killer and rapist. In the process of hunting down the killer, Soo-hyun slowly starts to lose all reason and will not listen to the pleas of his dead fiancée's family.