This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / Across the Universe
The army segment of "I Want You (She's So Heavy)".
Naked Vietnamese women who do a weird kind of gymnastic dance thing while standing in water about three feet deep, presumably near a beach... and then fall over dead for no apparent reason. Then we're treated to a lovely shot of dead naked bodies with water washing over them.
Lucy walking into Paco, the anti-war radical who was all about peace marches and giving flowers to cops, building bombs. It's even more horrifying that after Lucy calls him out ("I thought it was the other side that dropped the bombs") it doesn't affect him whatsoever. He just continues to build the bombs. Later on after that scene, Jude opens up a news paper and the headliner says that anti-war strikers were killed in a homemade bomb blast. Jude's horrified face when he thinks that Lucy was killed in there as well (she wasn't thankfully), and the Scare Chord that plays along with his expression says it all.
Orion's death is the nastiest in the series by far. After being frozen without the proper preparation, he comes out blinded, with severe tissue damage (squickily described as being like a wet sponge), and dies within minutes.