A character is on the verge of crossing a line that can't be uncrossed, but is wavering, when he suddenly receives a provocation that drives him to commit the act, or at least attempt to. A (usually) more lethal version of Fisticuff-Provoking Comment.
For instance, The Hero has a gun to the head of his Arch-Enemy, but is held back by his Thou Shalt Not Kill principles. As he struggles internally, his foe brags about stuffing his girlfriend into the fridge, and it's the last straw that makes him pull the trigger.
This doesn't necessarily have to involve a gun, or even killing. It can involve "pulling the trigger" on any action which (seemingly) cannot be taken back and will (usually) alter things in a dark way, such as declaring war or turning to The Dark Side. The only constant is that the character must be hesitating, and there must be a singular provocation that pushes them over the edge.
If a protagonist does this and it's a more idealistic work (or Status Quo Is God), it'll turn out not to be so irrevocable after all, or a third party will intervene to prevent them from actually crossing the line. If not, this is often a Start of Darkness, or at least It Gets Easier and Gaining the Will to Kill. Do not press the Berserk Button lightly.
See also You Wouldn't Shoot Me, which specifically applies to guns, and usually doesn't have the holder carrying through. Get It Over With, Strike Me Down with All of Your Hatred and Please Kill Me If It Satisfies You have the target trying to invoke this.
Compare Kill Him Already! when the provocation comes from the character's friend or ally. Contrast Villains Want Mercy. If they decide to pull an I Surrender, Suckers instead, that might have been a provocation enough for the hesitating character to "pull the trigger".
- Kanako's Life as an Assassin: The titular Kanako accidentally becomes a top-class hitwoman thanks to the fact that every person she kills reveals themselves as an Asshole Victim and makes her pull the trigger without any conscious effort on her part.
- Trigun: Vash the Stampede has had this forced upon him in hopes of breaking his pacifist spirit, indirectly or directly, by Knives, the most notable example being Legato who has the townspeople under his control. The only way for Vash to save them is to kill Legato, which is just what he wants, sending the former into a Heroic BSoD that lasts through the subsequent episode.
- Blacksad: Blacksad is about to kill Ivo Statoc, the Corrupt Corporate Executive who raped and murdered his former girlfriend, but still has his doubts whether he can really go through with a Vigilante Execution even after he already put Statoc's bodyguards in the hospital to get to him. Fortunately, Statoc's arrogant gloating about his own coldblooded nature and flashing a confident smirk angers Blacksad enough to pull the trigger.
- Tarzan: A failed example during the final showdown, when Tarzan gets his hands on Clayton's gun.
- Return of the Jedi: In the climax, Luke is hiding from Vader, refusing to fight him or be goaded anymore. When Vader susses out his sister's existence and brags that they can just turn her instead, Luke flies into a rage, starts channeling The Dark Side (which Yoda and Kenobi both told him was a point of no return,) and brutally curb stomps Vader. Fortunately, after cutting off Vader's hand, Luke manages to regain control of himself and pull back to the light side.
- Death Becomes Her: During a tense confrontation at the top of the stairs with a rejuvenated Madeline, Ernest snaps when she insults him and nearly strangles her, before leaving her tottering on the top step. She begs Ernest to pull her up and he begins to... Until she shouts "Hurry up, you wimp!" Ernest responds by poking her shoulder and sending Madeline down the marble stairs in a bone-shattering tumble (she gets better... Sort of.)
- Star Trek: The Next Generation: Probably happens, but left slightly ambiguous in the episode "The Most Toys". Data has been captured by a rich jerkass that intends to keep him as a collectors item, and spends the episode trying to break him into submission. Near the end, Data has turned the tables and has a very deadly, extremely painful weapon pointed at the guy, who starts taunting him for not being able to summon the rage necessary to kill. Data then calmly says "I cannot permit this to continue" and aims the weapon...and gets beamed up by O'Brien, who sees that the weapon is discharging and is able to dissipate it harmlessly. Data claims it must have malfunctioned during the teleport.
- Game of Thrones: Tyrion has a crossbow trained on his father Tywin for sleeping with Shae and for a lifetime of abuse. But he's wavering and Tywin doesn't believe he'll do it. And he really seems like he might not, especially after Tywin restrains himself from calling Shae a whore. Despite everything, Tywin seems to be winning Tyrion over...until Tyrion refuses to go back into the bedroom where he killed Shae and Tywin says, and promptly gets killed:
Tywin: Are you afraid of a dead whore?
- In the Cold Case episode "Justice", William holds a gun to the rapist of his sister, and shoots when the latter brags that all his victims loved it.
- The Magicians: In the episode "The Flying Forest", Penny ends up needing to get his hands sliced off as a result of them getting subjected to a curse. Quentin is reluctant to do so, but eventually proceeds to chop them off in rage after getting goaded into it by Penny.
- This turns out to be the reason Bryce was killed in 13 Reasons Why. After being beaten up by Zach and left by the river, Bryce is found by Jessica and Alex. He begs them to help him, as he can't walk or call for help, and if he's left out here all night he'll probably die of exposure. Even though Bryce is an asshole who has hurt him and everyone he cares about Alex can't bring himself to leave Bryce to die like this and helps him stand. However, when Bryce begins threatening violent retribution against Zach and accuses Jessica of setting him up to her face (he having raped her), Alex shoves him into the river and lets him drown.
- In Hamilton, Burr is enjoined in a duel with Hamilton, but is hesitant to shoot at his former friend-turned-rival, and is strongly considering wasting his shot in the air, confident that Hamilton will do likewise. But then he sees Hamilton putting on his glasses. Convinced that the only reason Hamilton would don glasses is to improve his aim, and refusing to let his children be orphans, Burr shoots Hamilton, killing him.
- Injustice: Gods Among Us: In the opening cinematic (in an alternate universe,) the Joker has Mind Raped Superman into destroying Metropolis, and killing Lois and their unborn child. When Batman is interrogating the Joker, Supes shows up, enraged. He grabs the Joker in a chokehold and is wavering over what to do, when the Joker makes a tasteless joke about how "maybe he won't kill his next family". It pushes him into tearing the Joker's heart out of his body and making himself an Evil Overlord of the Earth.
- Ensign Sue Must Die: Spock resolves to kill Sue, and has a phaser set to "Pocket Death Star" pointed at her, but is wavering, with his hand shaking. When she starts bragging about being a perfect marksman, he scowls and blasts her full-force. (But she turns out to be immune to phasers.}
- The Order of the Stick:
- In one strip, Haley gets cornered in a basement by the members of her former Thieves' Guild, under orders from the guildmaster to kill her. She starts talking about how difficult it's going to be knowing the names of all the former friends and comrades she's going to fight... but when she sees Toby, who runs the guild's dog-fighting ring, she remembers that most of the people in the guild are assholes, stops feeling guilty about hurting them and sinks two arrows into his chest.
- When Roy gets into a fight with a vampire who was raised from the corpse of one of his friends, the vampire attempts to demoralise Roy with an underhanded taunt about the accident that killed his baby brother Eric. This convinces Roy that the vampire isn't actually his friend, but a monster that has hijacked their corpse, and he starts fighting more fiercely as a result.
- One Sev Space comic (made before the release of Attack of the Clones) has Yoda telling Obi-Wan that he has foreseen that Anakin will turn to the dark side. Obi-Wan asks if it is due to a Sith Lord or emotional scars from losing his mother, and Yoda says it's worse than that, leaving Obi-Wan wondering what hideous evil could produce such anger... as it turns out, Jar Jar Binks is back.
- An early episode of My Little Pony: Friendship Is Magic has a group of buffalo planning a stampede to drive off the settler ponies in an overly-simplified allegory for Native Americans and settlers. The Mane 6 have attempted to talk them down, and at the start of the stampede, it seems that they've succeeded, as the chief of the buffalos hesitates to actually go through with it. Until Pinkie launches into a reprise of her friendship song, which the chief absolutely hates, causing him to charge into town.
- It is alleged that Tsar Nicholas of Russia was once agonising over signing a particularly controversial order, when an aide de camp approached him and tried to comfort him with the words "Majesty, we know how difficult it must be for you to decide"... not realising that the one thing the Tsar hated above all else was the idea that others perceived him as a weak, indecisive leader. If true, it was likely hearing these words that convinced him that he needed to sign the order... the year was 1914, and the order was the declaration of war on Germany that triggered World War One.