This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / Apocalypse Now
Where to begin? A wounded Vietcong begging for water, as he struggles to keep his entrails inside him with a pot lid. The over-the-top raid on a village where schoolchildren flee from the sound of approaching helicopters, which is then spectacularly napalmed. Hysterical "civilians" grenading MediVacs. A tiger out of friggin' nowhere. An abandoned MASH with corpses left to rot in lockers. A plane crashed in the water with no explanation. The massacre of the sampan. A remote outpost with no apparent leadership (presumably being the victim of an apparently well executed VC ambush), the unlucky men stationed there being slowly driven insane by fireworks, flashing lights and haunting music. Kurtz's compound. Kurtz placing Chef's recently severed (and still-twitching) head on Willard's lap and of course the Montagnard ceremony. The film seems to be trying to show the entire Vietnam War as being this trope, and it does a pretty good job of it.
The movie can be summed up with a single, nightmare-fueled quote: "I watched a snail crawl along the edge of a straight razor. That's my dream; that's my nightmare. Crawling, slithering, along the edge of a straight razor... and surviving."
Kurtz himself is something of this. The way he's almost always shown in the dark, and only ever partially visible, is particularly creepy, and one can only imagine how Willard must feel in his presence. Even worse still he's more than willing to decapitate an innocent man and casually drop his head in front of his commanding officer.
The REAL Nightmare Fuel? The worst part of it all might be that many veterans and witnesses have said that this film is one of the truest representations of what the war was like.
"There they were in a pile. A pile of little arms."
And while Kurtz is telling this story the camera cuts back to Willard while at the background you see one of his guards as a vague silhouette in the distance very slowly moving and turning around. When you first see this it's a veritable unnerving Jump Scare, especially since it's not clear what he is doing back there? It seems as if he is catching a fly or something?
The collective conclusion of all the terrible things Kurtz had faced in his life and what it turned him into, summarized by his final words, "The horror.......the horror". He doesn't even say it as dramatically as people would think, he's so shell shocked he's gone way past that.
The execution of the water buffalo is genuinely disturbing to watch given that at one point it looks directly at the camera.
Willard is trying to find someone in charge at a chaotic, violent battle over a meaningless bridge upriver. "Yeah, I know who's in charge," a Scary Black Man finally tells him after a particularly brutal act on the battlefield. And then he just glares at Willard, letting his silence confirm exactly what he's talking about.
Kurtz domain is littered with corpses and dismembered head and his group of fanatic just waltzing around, it's clear they can kill Willard's squad but they just stares at them.