First, let me preface this by assuring the reader that Full Metal Jacket (which I'll call FMJ from here on out) is one of the greatest horror films of all time. Many people would claim that FMJ is not a horror film but a war film and they would be wrong. FMJ is one of the most realistic horror films ever created that documents, in stark visuals, the transformation of innocents to monstrous killing machines and the consequences of unleashing them upon the world. It relays with an unblinking eye man's inhumanity to man and the historical bloodlust of mankind for unceasing war and conflict. It is the quintessential horror film that relays its message not with symbolism or subtle references but with a rifle butt to the face.
1: R. Lee Ermey
If there was ever a more perfect character in a film than Gunnery Sergeant Hartman, I haven't seen it. R. Lee Ermey was the most potent, over the top encapsulation of the entire film's zeitgeist in one unforgettable character. He was the symbol of war itself: in a polite, genteel society, the populace would be ashamed of something so crude, base and uncivilized but when the inevitability of conflict came and the threat became too personal, society would cry out for such animalistic, brutal men to protect them. He was the devil himself but the necessary devil that society never admits it needs but always turns to in case of danger. The closest character I can compare it to is John Wayne's character Ethan Edwards in John Ford's The Searchers.
With 99% percent of the greatest lines coming from R. Lee Ermey's character, most improvised by him, you could say this movie was the R. Lee Ermey show and you wouldn't be wrong. But there are plenty of other great lines from the film that my friends and coworkers reference non-stop to this day. It gets to the point that when a new person comes onboard and is driven insane by our constant references to certain movies, they break down and watch them only to start quoting them themselves. Between Monty Python and the Holy Grail, Caddyshack, Full Metal Jacket and The Simpsons, you pretty much have a movie quote for every situation in life.
3: Two act play
Full Metal Jacket is essentially two movies squished somewhat uncomfortably together, but it's that juxtaposition that makes the whole greater than the sum of its parts. The first act is the creation of the monster, a striking success in the case of Joker and a disastrous failure in the case of Private Pyle, but an unblinking portrayal of dehumanization and stripping away of innocence and its replacement with unadulterated savagery. The second act is the monster escaped: a spinning, dizzying dance of the macabre as the monster's creator cackles with delight as his creation is unleashed upon the world. They are like quarreling siblings whose personalities are so polar opposite it leads to constant fighting but, in the end, they are inseparable and imagining one without the other would be intolerable.
4: The Soap Party
While most people would say the most brutal scene in the movie is the bathroom scene where Private Pyle's and Hartman's story lines end, I would say a far more brutal scene is the soap party where Private Pyle's company mates hold him down and beat him mercilessly with bars of soap wrapped in socks. Unlike the bathroom scene, the brutality here was more than just graphic and gory; it was merciless, compassionless, inhumane and was a breaking point where Joker's humanity left him and he became the monster of Hartman's design. The closest analogy I can think of is a puppy with an innate defect that makes him unable to stop peeing on the floor and the owner, who has been concerned and caring up to this point, embraces the darkness and beats the puppy mercilessly out of frustration. This is the image that came to mind as Pyle is lashed to the bed with the sheet, unable to escape and lets out soft whines and whimpers as the blows rain down upon him. He is now totally alone, completely abandoned, his only supporter now his torturer, and he has become eternally trapped "in a world of shit".
5: The sniper scene
This is the scene that sticks with most people and for good reason. It is the climax of the film, the encapsulation of the film's message, and one of those rare scenes that brings the viewer to an uncomfortable place where they start to feel that pull of savagery, that transformation to the base impulses of humanity they just witnessed on screen with horror. A sniper has just tortured and butchered characters we have come to love and sympathize with; when they finally corner the sniper it's a 12 year old girl who was sacrificially left behind to slow their advance. And instead of a little girl, terrified and praying, we see a butcher, a destroyer and our impulse is "Kill her, make her pay for what she just did to our friends". We have become the monster, we have been transformed, we have become what we would never allow ourselves to be. A fake war in a fake story in a fake scene has made us feel things we never thought we could feel and never admit we could contemplate. And the stark contrast between the cold, remote, detached killing wracked by the sniper and the personal, intimate killing of the sniper herself is a masterful juxtaposition that makes the psychic shock even greater.
Bonus trivia: R. Lee Ermey was never supposed to be in the film. He was originally just a hired technical consultant but wanted the role so bad he would dress up in his drill sergeant uniform and literally transform himself into the character on set. Stanley Kubrick swiftly noticed and gave him the part he was born to play. The role of Gunnery Sergeant Hartman was originally given to Tim Colceri who was eventually cast as the insane helicopter door-gunner. Colceri never forgave the betrayal and in a later interview wept bitter tears over losing the role.