A necessary part of the Tragic Monster is the part where the ordinary person becomesthe monster, despite all their resistance to the change.
Our tragic transformer of course despises the thing they become and all the violence and destruction that comes with it, and hate the fact that they hurt people on a regular basis.
Generally, when the change happens (usually by surprise somehow), they'll be with someone they care about, and, resisting the inevitable with all their might, they'll barely have enough leftover strength to growl a pained "get away from me!", and/or run away from anyone they might attack, before the generally painful change happens in full force.
This is a staple of modern werewolf portrayals and characters who have a Super-Powered Evil Side that doesn't come out right away. They tend to have this kind of scene at least once, and may have them all the time.
They will stubbornly resist the monster that they are turning into, trying in vain to stay in control of their mind and body lest they go on a terrible rampage. After an appropriately dramatic fight, they fail, and the monster is released (often going after the person they were with at the time who, of course, failed to follow the order to get away), though not before we get a full scene showing them fighting tooth and nail against their darker side. That is, at least until it wears off (until the next time) or someone, generally another person they care about, is forced to stop them.
Occasionally someone might succeed in fighting the change, though it's usually of a "I can't hold it off for long" nature.
See also "I Know You're in There Somewhere" Fight, The Mind Is a Plaything of the Body, and Enemy Within. Generally characters who go through this a lot are full of self loathing and may be Death Seekers, or make moves to keep themselves under lock and key, so that they can't hurt anyone anymore. Often accompanies a Face Monster Turn.
In the manga Claymore, the warriors can gain immense power by tapping into their Super-Powered Evil Side, but need to resist its temptations or go too deep in and become a monster themselves. Those who manage to pull themselves back from the very brink of going to the dark side and "Awakening" gain a boost in power and can tap their power more freely than the others.
Bruce Banner is usually lucid enough before he turns into The Hulk to tell innocent people to get away and try to quell his anger before it's too late. On a very good day, he might succeed, only to be set off by an even bigger event.
Doctor Curt Connors of Spider-Man, being another character influenced by Tragic Movie Monsters, has lots of scenes like this as he's turning into the Lizard. He failed in the worst way possible when he devoured his own son. Curt effectively died after that, leaving only the Lizard.
Morbius the Living Vampire, though always visibly a monster, faces this trope psychologically when his bloodlust hits him in an inconvenient time.
Jack Russell, the titular werewolf in Werewolf by Night, often goes through this during full moon nights. He is capable of delaying his Involuntary Transformation for a little while, but the longer he does so, the angrier his werewolf self is when he inevitably does transform.
Johnny Blaze has this problem in Ghost Rider: Spirit of Vengeance. Since the Rider in this movie would kill for the pettiest sins, he constantly struggles against its influence with isolation or stolen meds.
Sam Vimes is a rare case of someone who succeeds in keeping the beast locked inside. It's not an actual werewolf-like curse, but his darker nature that's always straining to be released.
In Thud!, Vimes gets possessed by a vengeance demon. While he starts going on a rampage, he prevents himself from killing a squad of defenseless dwarves long enough for Angua to take him down. To show how awesome this is, it's noted that if he had done so for just a few seconds longer, his muscles would have torn themselves apart.
Vimes also does it with the Gonne in Men at Arms, as opposed to Carrot who is The Immune.
Otto Chriek and other vampires in the Uberwald Temperance Movement have forsworn drinking human blood, but often have to struggle to suppress their natural urges (they generally succeed by diverting the urge into some sort of addiction or hobby). Though if one cannot fulfill their new addiction they will go berserk.
The Red Court vampires in The Dresden Files don't turn into full-blood vamps after being infected until they give in to hunger and lethally feed on a human being. Some half-bloods are known to have resisted the temptation for centuries.
Dillon Cole wrestles with his destructive urges in Scorpion Shards, gradually succumbing to them and unwittingly feeding and strengthening the otherworldly parasite corrupting him.
The Hunger Games: In Mockingjay a hijacked Peeta spends a large section of the book fighting the urge to kill Katniss, asking the others to kill him, refusing to have his handcuffs removed and using the pain of the too tightly wound cuffs to keep himself sane.
This trope plays out a number of times with Oz, a werewolf, in Buffy the Vampire Slayer. Before his status is out in the open he tries to get Willow to leave without telling her why (fearing he's going to be killed after having overheard Buffy say she's going to off the werewolf when she finds it). Later, in "Beauty and the Beasts", he even tells her to get away from him after he's been locked up in a cage.
Similarly, Angel is shown having to resist feeding on humans in the episode "Angel" where his vampiric nature is first revealed. Darla attempts to tempt him into feeding on a wounded Joyce.
In the Doctor Who episode "The Pandorica Opens", Rory is unable to stop himself from shooting Amy when his programming as an Auton takes over; fortunately, both of them get better in time to help the Doctor save the universe.
Forever Knight used this a lot, with Nick trying to resist losing control of himself and killing someone.
In the Supernatural episode "Meet the New Boss", Castiel is about to be taken over by the Leviathans, and yells at Dean and Bobby to get away from him.
Happens in the werewolf segment of the movie within a movie in Thriller, as one of many send ups to classic horror movies.
The vampires in Vampire: The Masquerade literally have to resist the Beast, where "Beast" is a lore term for their compulsion to just let loose and feed upon any human within reach (usually breaking The Masquerade in process). To resist it, you have to pass Willpower checks every time the character experiences a particularly traumatic event, such as being exposed to fire or even sunlight. Having high Humanity score also helps. Fail one too many checks and your character turns into a mindless bloodsucker, though.
In the spiritual successor, Vampire: The Requiem, the Beast is still present and at war with the Man. However, various vampire groups have developed ways to ease the struggle. The Ordo Dracul have a Coil of the Beast that makes the struggle simpler (at its final level, you can give in for an hour to defeat it for the rest of the night), and the bloodline known as the Kukufuuji have developed a discipline based on restraining it.
The second Ouendan game has the teenage werewolf Ooyama Garou try to avoid staring at round objects while on a date, as that is what triggers his transformation. Unlike most examples, this is played for comedy and his trigger are things like balloons, baseballs, and even the shiny bald head of his boss. Assuming the player wins the level, his girlfriend is totally unfazed by his werewolf form because she's a dog lover. And what sets him off at the end? Her eyes.
In the worgen starting zone in World of Warcraft, you're sent to talk to this one guy and he growls at you to stay away, then loses control and attacks you, at which point a teammate of your comes in and shoots him. You shrug off the resulting wound as 'probably nothing', and say nothing about it even when it starts festering and you're feeling quite ill.
In Resident Evil: Code: Veronica, Steve Burnside becomes a hulking brute with an axe. After a bit of hack-and-slash, he manages to resist the murderous impulses of infection, at the cost of his life.
The plot of the Flash game Don't Escape. The hero is a werewolf who realizes he's only got a few hours before the moon rises, and must try to restrain himself as best he can.
In Melty Blood, Vampire Sion/Sion TATARI (Sion who had given in to her vampiric impulses) is a playable character. In contrast to regular Sion's collected and stoic composure, V. Sion/TATARI is more noticably emotional in her speech - several of her pre-fight quotes are either screams of pain or halting pleas for whoever she's fighting to run away.
In the same vein, vampires in the webcomic The Kingfisher risk mindless blood frenzy if they don't drink blood regularly. This isn't the focus of the story so it doesn't come up much.
Several times in BIONICLE. Most of the 2005 storyline revolves around The Toa Hordika wrestling with their Beast Man nature. Also, Lewa had to do this during the time he was possessed by a Krana.
Happens several times in the Jackie Chan Adventures episode "The Lotus Temple," where a little girl is cursed to become a monstrous guardian for a magic temple whenever someone intrudes. Since she's very opposed hurting anyone, she does this near every time. The heroes manage to find a loophole (being invited doesn't count as intruding), and at one point she does manage to stop herself from going psycho long enough for them to use it. Later on, she's able to focus herself towards the real villain.
Some Serial Killers have tried to get themselves arrested or institutionalized as a way of stopping themselves from murdering people. Sometimes it works. Other times...