The fifth opening theme "Sugar Sweet Nightmare" is a bit unnerving. One scene in particular is when human hands burst out of the mouths of cats and roughly grope Tsubasa's naked body as she tries to run away from them.
There's also one of the final scenes in the last episode. The cat licks Koyomi's neck and his tongue is sharp enough to cause Koyomi to spout blood out of his neck.
Kisshot's appearance at the end of Mayoi Jiangshi. She doesn't do anything particularly horrible onscreen but still, creepy.
The beginning of the Nadeko Medusa arc starts off with Snake God Nadeko stabbing Koyomi, whose body is already torn up from her previous assault, and giggling creepily in a shower of his blood. The entire arc is her descent into insanity.
The truth of Sodachi's mother's disappearance. Due to her mental issues, she was unable to admit to the fact her mother starved to death and thus spent two years caring for her corpse. Only once it decomposed beyond any recognition did she register her mother as "gone". Brrr.
The ending of Koyomimonogatari consists of Gaen calmly walking toward Araragi while talking about how to stop the darkness that engulfs the city... Before casually pulling the Kokorowatari and slashing Araragi into literal pieces that scatter into the air. And take note that this is a rare occasion where the series defy exaggeration from Unreliable Narrator because the narrator himself is in shambles now. This is what actually happening onscreen.