The nightmares Tigerstar sends Lionblaze of himself slaughtering Heathertail over, and over again during Power of Three. One features a river running with her blood, and in another he slices open her throat and blood comes pouring out until he is completely drenched in it. In another one, it sounds almost like he tore her open.
The way the Dark Forest cats train themselves in death blows: violently killing each other in front of an audience, repeatedly . In one particularly jarring scene from Omen of the Stars, the chapter suddenly cuts off with Jayfeather gagging as he hears one of these unlucky cats screaming in agony when his belly is slit open.
In the manga at the end of Crookedstar's Promise, the Slasher Smile Mapleshade has makes her look like a demon.
Many of the fans know that Mapleshade may be completely psychotic, and they want to see a Super Edition based on her backstory. The thing is, the authors have said that the reason they won't do a Mapleshade Super Edition is because her backstory is too scary to recount. Many fans speculate it may have something to do with cannibalism, though reasons for this are unknown.
Beetlewhisker's fate in The Last Hope. It involves Brokenstar lifting him up from the ground by his throat while the poor cat cries for help, until his neck snaps. The worst part is, Starclan can't get to him, since he died in the Dark Forest, and the Dark Forest doesn't want him. Basically, his soul is trapped inside his body, to forever live in the pain of dying.
Rock and The Ancients
Rock. Just... Rock. He's a hideous bald cat with bulging blind eyes and curled up, untrimmed claws. Not to mention the fact that he oversees a test which involves navigating through a complex underground maze in complete darkness. Oh yeah, and if it rains, the tunnels flood and you drown. Good luck.
In Night Whispers, Rock is the last thing that Flametail sees before he dies. Sweet dreams.
In one part of The Last Hope, Rock gathers up all the Ancients to ask the Three why they have been abandoned. If the perpetually sweet Half Moon screaming like a maniac isn't enough to terrify you, Rock starts trying to bury the Three alive.
The ending of Sunrise. A combination of Hollyleaf's insanity, including trying to murder her own mother, Leafpool being pretty much suicidal, Sol in general, or the feeling of despair that emanates from it makes it pretty unnerving.
In one scene, Hollyleaf imagines a mouse as Leafpool and violently tears her body to shreds, reveling in the feeling of ripping the life out of her.
In Dark River, Lionblaze has a dream where Hollyleaf transforms into a fox and brutally murders him.
In Sunrise, Jayfeather looks into Hollyleaf's mind and sees her murder of Ashfur. She treats him like a piece of prey to be killed for her Clan.
When the entrance to the tunnel opens up in Sign of the Moon, Lionblaze imagines Dovewing and Icecloud falling to the ground and finding Hollyleaf's decaying corpse.
Jayfeather's dream about Hollyleaf in Sign of the Moon.
Starclan and Visions
The Bonehill. The. Freaking. Bonehill. Does anyone blame Firestar for freezing in terror when he sees the thing? Not to mention the scene that follows...
Also note that The Bonehill in Firestar's dream apparently had some cat bones in it. Now think about that for a second, where did they get them? They couldn't have raided the graves of long-dead cats because they don't mark where they bury other cats, and if they were buried underground, they would have turned brown, and the book described them as "sun-bleached". I can only think of one answer: cannibalism.
Bodies left above ground will be eaten by other animals, like ants and various birds. It's not much more pleasant, of course.
Also, whenever StarClan says they're going to give up, like in Eclipse, and the end of The Fourth Apprentice, when there's going to be a freaking war between two groups of dead cats.
The story in Code of the Clans about how Code #14 (a warrior does not need to kill to win battles) came to be. The medicine cat is visited in her dream by a very young apprentice, essentially a child, who was killed in battle the day before. It's eerie enough with him reminding her that he's dead, and speaking with a wisdom beyond his age. But then he fades away (his eyes being the last thing to disappear), giving us this line as he fades: "That WindClan warrior didn't need to kill me. I knew I was beaten. If he'd let go of me, I'd have run away. He didn't have to keep biting me, harder and harder..."
Fireheart's vision of a faceless queen in Forest of Secrets.
This is comparatively small next to the others, but in Skyclan's Destiny, in the prologue, the cats couldn't carry back the dead bodies of two warriors back to the gorge for the fear of being killed themselves. One of the elders gets upset at this, and responds by hobbling off towards a cliff face. Spiderstar stops him and says, "We've lost enough warriors tonight." This implies that the elder was going to commit suicide.
At the end of Rise of Scourge, the picture of Tigerstar there and Scourge crawling all over his body.
Silverstream's death. I know that Death by Childbirth isn't particularly rare in literature, but I remember that when Jayfeather was walking in Graystripe's memories, his narrative described her body as being surrounded by blood. I think in Long Shadows the words were, "Her lifeblood gushing onto stones as she gave birth to a pair of tiny kits."High Pressure Blood and Death by Childbirth isn't a pleasant combination...
This really comes through in the (multiple) sections of the books featuring High Pressure Blood, most notably Tigerstar getting ripped open and writhing in agony as he bleeds to death nine times in a row, Brightheart and Swiftpaw being brutally mauled by a pack of dogs, Sharptooth slaughtering and eating Tribe cats, Firestar's nine lives ceremony, and every battle involving Lionblaze.