In Footloose, Reverend Moore was forced come to this realization when the adults in his town takes his advice to destroy literature that led to the corruption of youth too far, even for him.
In Braveheart, Robert the Bruce goes through this after he sees the look of despair on William Wallace's face when Wallace sees that the Bruce betrayed him at the Battle of Falkirk. It doesn't sit well with him, and he vows later never to be on the wrong side (meaning the English) again.
Philip in Pirates of the Caribbean: On Stranger Tides, when he realizes that Syrena was pulling him to safety, not attacking him, when he captured her.
According to Saw director Darren Lynn Bousman, Saw III was intended to have Big Bad/Well-Intentioned Extremist Jigsaw have one of these moments after seeing the results of his legacy:
"For the first time, we actually see him break down and cry. Imagine your entire life's work. You're on your deathbed. You know there's nothing else you can do and here's how you'll be remembered: as a killer, as a murderer. Not as someone who helped people. Not as someone who changed lives. Someone who took away lives. The one thing he didn't want to be and, as he's on his deathbed, he's realizing this."
You can still win with the classics. In the climactic scene of The Bridge on the River Kwai (seven Oscars!), Colonel Nicholson (Alec Guinness) realizes that he has been collaborating with the enemy to build a bridge out of sheer egotism. His final words as he dies and the bridge blows up are "What have I done?" The fact that he says it in a shell-shocked murmur only makes it all the more poignant. The last line of the film, by the only surviving major character, answers the question: "Madness!!! Madness!!!"
Fred in the The Flintstones movie, when he realises he's laid off every last one of his friends.
X-Men: The Last Stand features Magneto saying the line, when he finds himself on the other side of the Mutants Vs. Humans war he's been pushing for and Phoenix finally goes crazy and starts killing people.
In the film version of Silent Hill Daliha, played as a much more sympathetic character than in the games, says this word for word. minus the "my god" part, when she gives up Alessa to her psycho sister, Christabella to be "cleansed."
In The Fourth Protocol the traitor Berenson utters this when he realizes that the 'South African' to whom he has been passing NATO secrets is really a Soviet agent. SIS chief Sir Nigel Irvine (Ian Richardson) quietly responds "You've betrayed your country."
Home Alone: Kevin's mom, when she realizes she left Kevin at home while the family left for Paris.
"What kind of a mother am I?"
V for Vendetta: This phrase is written in the diary of Dr. Diana Stanton, aka Delia Surridge, who had tested the effects of experimental biological weapons on cultural minorities in a post-apocalyptic Britain. She realizes the error of her ways when confronted with an escaped test subject and the destruction of her work. For his part, V knows she feels this way and gives her some mercy by killing her with a painless poison compared the gruesome Karmic Deaths he arranged for others.
Delia Surridge: [V gives her a rose] Are you going to kill me now? V: I killed you 10 minutes ago. [shows her a hypodermic needle] While you slept. Delia Surridge: Is there any pain? V: No. Delia Surridge: Thank you. Is it meaningless to apologize? V: Never. Delia Surridge: I'm so sorry. [dies]
Similar. When she wakes up in the middle of the night and realizes V is there, she says "Oh thank God. Thank God." and breaks into tears.
In Memphis Belle, tail gunner Clay Busby agrees to let co-pilot Luke Sinclair log some time on the gun so he can try and shoot down a German fighter of his own. Later during the approach to Bremen, Luke gets his chance and scores a kill. At first, he whoops with elation ... until the downed Me-109 crashes into the rookie-crewed bomberMother and Country. All Luke can do is scream in horror, wide-eyed, as he watches the two halves of Mother and Country tumble earthward. He remains in a Heroic BSOD for some time after.
Matt Stover's novelization wisely leaves out the Big "NO!", adds a fleeting moment of Never My Fault, and rather than throwing a tantrum has him immediately try to kill Sidious—but he's lost so much of his power that he can only destroy droids and equipment, he can't even touch Sidious—and in the end he doesn't want to, because now this is all he has left. The only person who will understand, and forgive, and gather him up.
In The Mummy, at the first sign of Imhotep's resurrection; a spontaneous plague of locusts; the Egyptologist says, "What have we done?" as he's covered with the damn things.
This happens on two different occasions in What's Eating Gilbert Grape?. At the beginning of the movie, Arnie - the mentally challenged young man - kills a grasshopper by beheading it, and then cries to Gilbert about it. Later on in the movie, Gilbert loses his cool - leading him to slap Arnie around, after being so protective of him.
Gene Wilder gets one in Young Frankenstein. On the DVD commentary, Brooks says this was to keep the film in the structure of classic Yiddish theater, where act two always ends with either "What have I done?" or "She's pregnant!"
Hilariously subverted in A Dog's Breakfast. Patrick wants to kill his future brother-in-law, Ryan. Ryan offers to help him chop wood by holding the wood steady for him. Patrick lifts his axe up, clearly intending to render Ryan in two. We cut to a shot of the woods just as we hear a dull thunk, complete with blood curdling scream. We then cut back to Patrick's face who whispers: "What have I done?".....only to watch him collapse in pain as we realise that he's actually hit himself in the leg with the axe's handle by accident.
In Repo! The Genetic Opera, Nathan says this twice after confessing that his daughter is not sick, rather he has been poisoning her in order to keep her from ever leaving him, once without the My God, and the last time with the My God.
In TMNT (2007), during Raphael's fight with Leonardo, he manages to break Leonardo's swords, and then pin Leonardo down to the ground, and then, he realizes what he's doing, and runs off, only to come back to save Leonardo from the Stone Generals, but fails miserably.
In Fluke, after Fluke attacks Jeff whilst the latter is driving, causing them to crash, he finally recovers his memories, and realises that when he was alive as a man he'd been quite the Jerkass to Jeff, and it wasn't Jeff's fault that Fluke/Tom had died. This later fuels Fluke's I Want My Beloved to Be Happy stance.
Event Horizon. Baby Bear is released from his trance just as the airlock is about to open. His reaction upon realizing where he is is upsetting, to say the least.
The Mist. The protagonist shoots his son, two elderly people and the heroine after realizing that they have nowhere left to run, but runs out of bullets before he can off himself. After getting out of the car to let the creatures kill him, the military then comes from behind and cleans out the mist, effectively ending the whole ordeal. Had they waited a minute longer, there would've been no need for them to die. Oops.
Avatar. The look on Selfridge's face as he watches the destruction of Hometree shows that he finally realizes what an asshole he is. Despite this, he still doesn't make a Heel-Face Turn.
After his Batman Gambit is pulled off succesfully, and the title character of Fresh ends up with what he wanted for himself and his sister, he goes to meet his father, and just starts crying uncontrollably. Understandable, since although most weren't nice people, he just got about 10 people killed as part of the gambit. And he is 12 years old...
A variation occurred in Kung Fu Hustle, where Sing tried to steal from the Cute Mute ice cream lady, only to realize that she was the little girl he tried to save when he was a kid. Then he ran away from her, ashamed at what he was trying to become.
Star Trek III: The Search for Spock plays with the trope with James T. Kirk saying the words after ordering the original Enterprise to self-destruct, and watching it burn up in the Genesis Planet's atmosphere.
Kirk: My god, Bones. What have I done? Bones: What you had to do. What you always do. Turn death into a fighting chance to live.
Given what his ship means to just about everyone in Starfleet, and especially to Kirk himself, it's easy to argue yes; he destroyed the one thing apart from his immediate crew and his son that he actually loves. Didn't help that he performed the act as part of a Roaring Rampage of Revenge over David's murder.
Terminator 2 does this (but doesn't say it) to Sarah Connor. After spending the majority of the movie as a fundamentally screwed-up person, she tries to murder a man, not because of anything he's done, but based on what he will do in the future. Everything from her attitude during the "hunt" to the weapon she uses is eerilyterrifyingly similar to the T-800's targeting of her in the original film. It's only when she's holding the gun to his head, in front of his terrified, sobbing wife and son, that she starts to realize exactly what her fear of the future has done to her.
Miles Dyson has a preemptive one when he is told about Judgement Day and the Machine War.
"I think I'm gonna throw up."
Wall Street: Bud Fox's father is the union manager for an airport which Gordon Gekko intends to buy, using Bud's help to seal the deal. Bud has one of these moments after realizing that Gekko is in fact going to fire the workers he said he would let stay.
In Schindler's List, Oskar Schindler, having turned down a request by a Jew to accept two of her relatives for not being skilled manufacturers and yelling to Itzhak Stern about how popular his factory is becoming as a "haven", calms down, reaches into his pocket, and pulls out a slip reading "Perlman"—the very same name attached to those he had just turned down. He reads the name, realizing his mistake, and has Stern make sure the two are brought to the factory, presumably to start training in manufacturing before becoming full-fledged workers.
Verbatim in The Rock after Goodspeed and Mason pull General Hummel out of the mess with Frye and Darrow. Hummel relents using the trope word for word before telling Goodspeed the location of the last rocket ("Lower lighthouse!").
In the made-for-tv Elvis Meets Nixon, it's 1970, and Elvis has snuck out of Graceland, out on his own for the first time in over a decade. He has fond memories of LA's Sunset Strip, but when he gets there he's appalled to find it overrun with hippies and head shops where his records are back in the 'golden oldies' section. A hippie recognizes Elvis and excitedly tells how he changed his life - in an irony worthy of The Twilight Zone, he points out that Elvis's spirit of rebellion created the counterculture. Elvis is horrified.
Peter in Spider-Man after he hits Mary Jane in his symbiote induced rage.
Played with in The Avengers: Loki uses his powers to Enlighten several agents including Dr. Selvig who activates his master plan (Selvig builds a machine that allows the Chitauri to invade Earth). When he is free, the Black Widow finds him at the edge of Stark tower, as if he's about to jump off. However, Selvig reveals that he knows exactly what he has done, and he knows how to stop it, climbing to the edge to point out that Loki's staff is the means to stop the machine.
Also, after Hawkeye is freed from his mind control, he asks Natasha "How many agents," wanting to know what he did under Loki's power.
In Pumpkinhead, a man makes a deal with a witch to summon a demon to avenge his son; who was killed by careless teenagers. He feels the pain each of the demon's victims feels, and realizes the horror of what he did. He then sets out to try and stop the demon from murdering the rest of the teens.
Scanners III: The Takeover. When the Psycho Serum she's been using wears off momentarily, Helena realizes what she's done and is distraught, but takes the drug again. She's also like this just before she commits suicide at the end.
The Mayor in Jaws has one of these moments following the shark attack in the estuary when he realizes that it was his fault for insisting the beaches stay open.
In Dennis the Menace, Mr. Wilson becomes filled with guilt when he learns that Dennis ran away because of his yelling at him earlier.
The Captain suffers two of them in The Lone Ranger. The first one is when he realizes he was duped into leading his men to killing innocent Native Americans. The second one comes after he stabs the Comanche Chief and sees his blood on his hands, calling back to the Big Bad claiming he had blood on his hands from the first incident.
Elysium: Delacourt refuses treatment from Frey after she's stabbed in the neck, murmuring 'no' in remorse for what she did.
The Hobbit: The Desolation of Smaug: The movie ends on a cliffhanger, as Thorin's efforts to take out Smaug fail and Smaug flies away to destroy Laketown. The very last scene is a horrified Bilbo whispering, "What have we done?"
Boromir has a massive one after he attacks Frodo to take the Ring.
Gríma seems to believe that Saruman is biting off more than he can chew and that he might be able to play both sides against the middle - right up until Saruman shows him the magically frenzied 10,000+ superhumans in plate armor. The look on his face is priceless.
Frodo has a "What did I almost do?" moment in The Two Towers. Under heavy influence from the Ring, Frodo doesn't notice that the Witch-King of Angmar is swooping in to grab him. Sam pulls him out of the way, simultaneously trying to get the Ring off. In response, Frodo pins him and threatens him with Sting. Sam manages to talk him down, but you can clearly see the horrified expression on his face as he drops the sword.
Permission to Kill ends with Curtis, after successfully completing the assassination of Diakim, breaking down in the middle of a recording of his eulogy for Diakim.
Curtis: [through tears] What kind of people are we?
Young Guns 2 Billy begins to regret his actions after Tommy dies. It becomes much more apparent after Chavez is killed, to the point where Garret finds him depressed and alone, and not willing to fight back.
The Film of the BookA Series of Unfortunate Events. Justice Straus is cast as a judge for the wedding scene in Olaf's play in the climax of the film. Being painfully naive and gullible for a judge, she doesn't realize that she's only there to officiate a wedding between Violet and Olaf, making it legally binding and granting Olaf control over Violet's inheritance when she turns 18. When Olaf reveals that the marriage was made legal entirely through Straus's involvement, she invokes this trope verbatim before tearfully running off stage, apologizing to Violet for not knowing better.
In The Quiet Man, Sean Thornton was a boxer who accidentally killed another fighter during a match. A flashback shows him staring in anguish as the ring doctor examines the fallen fighter then places a towel over his face. Afterwards, Sean quit boxing and vowed never to fight again.
Ian Richardson responds to this question with an epic "The Reason You Suck" Speech in The Fourth Protocol (1987). He plays Sir Nigel Irvine, a British Intelligence spymaster who's uncovered a politician passing NATO secrets to South Africa. Sir Nigel reveals that this is actually a False Flag Operation with the information going to Moscow Centre.
Politician: Oh my God...what have I done?
Sir Nigel: You've betrayed your country. You've passed untold numbers of secrets to Moscow, and endangered the lives of British men and women. And I'd say you've weakened NATO, perhaps irretrievability. Just you, and your schoolboy politics, and your idiotically conceited faith in your own importance.
Into The Storm: Winston's face when he sees the sheer loss of life he brought upon speaks more than a thousand words.
In Godzilla (2014), the look on Joe’s face after his wife's death says it all. His obsession is also most likely fuelled by guilt.
American History X: Henry's reaction immediately after he shoots Danny, as he realizes that he just murdered someone and blew his own life away for something as minor as receiving a dirty look from a racist.