This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / Ratman's Notebooks
In the book, the death of Socrates is so graphic that it arguably approaches Gorn
Finding Mr. Martin's remains.
The ending, where you suddenly realize that the entire thing is now an Apocalyptic Log.
The alternate version of Mr. Martin's death in the 2003 movie's DVD. Second Unit Director James Wong said, at the time, that it was the most disturbing thing he'd ever shot in his entire career and that he actually got sick while filming it. It really should not be watched by the faint of heart.
In the book, Ratman finally snapping and vowing revenge on Mr. Martin is depicted in very unsettling and emotionally-raw way.
Michael Jackson's theme to Ben becomes this once you realize what the context is.
In the 2003 version, the rats attacking and killing the cat.