Sweet, soft-spoken Ayano punching Misao in the face in Chapter 3 when under the influence of a Demon Brand, and screaming that she hates her and never wants to see her again... brrr.
Kagami's fall and transformation in Chapter 9, and Kazumi's sudden death immediately after. She's just... gone.
The Third, which takes the form of a giant, rabid, emaciated three-eyed dog with a stomach filled with maggots, is terrifying enough already, but it gets turned Up to Eleven when it swallows Kagami whole.
Remember the Yuki cosplayer in that one episode of Lucky Star, the one who worked at Konata's Cosplay Cafe? She gets flayed to death by monstrous crows in Chapter 20, leaving a completely skinless corpse behind.
In My Sunshine, Mami has her memories eaten until she is basically reduced to an infantile state of mind. It's both scary and heart-wrenching.