This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / The Mentalist
Any crime scene involving Red John. Other serial killers are terrified of him.
In a season one episode, it's all but said aloud that one of Jane's former clients killed herself because Jane made an error in his cold-read and touched on a lifetime of trauma with the wrong phrase.
The season finale "Red John's Rules." During the episode: 1) we are told that Jane has list of Red Johns suspects down to seven people and 2) we find that the victim of the week was someone Jane had known as a child, and this woman represented a happy childhood memory for Jane, a small one he had never spoken to anyone about. During last five minutes we are presented with a disk given by someone who knew Red John. In it, One of Red John's victims reads a message from Red John to Jane. Red John says that he knows that Jane has narrowed down the list to seven people. Red Jon lists all of the names. They are exactly who Jane had picked as suspects, and as Red John reads the names, Lisbon puts down their photographs across the laptop. Then Red John says that he killed the victim BECAUSE she was a happy memory from his childhood, and the murder would lead directly to this disk. Lisbon ejects the disk, and Jane snaps it to pieces. Neither of them speak through the whole thing. Absolutely chilling.
The ending of "The Desert Rose". Lisbon gets a distress call from an abandoned building and back up is running late. She goes in. She finds Partridge (one of Jane's suspects) right before he dies and gets pulled back by the hair by an unknown assailant. Red John then calls Jane and tells him that Lisbon can't get the phone. While drawing a smily face on Lisbon's face. With Partridge's blood.Sweet Baby Jesus...