Thousand Shinji: any scene involving Rei's worship of Nurgle. Especially the Iruel battle and Rei's transformation into Reigle.
ARSENAL: The Black Bunnies, both what they do and what eventually gets done to them. Some of the other characters, especially Shinji, can edge into this as well.
In Axis Powers Hetalia there is a comic, which later expanded into so many spin-offs it's practically a fandom of its own, known as the World Financial Crisis Gangbang. The cast blames America for the economic crisis... so they (with the exception of his little brother Canada, thank god) gang rape him. And that's not even going into the "America's revenge" fics, where he makes it his priority to torture and murder his rapists in the worst ways the author can think of at the time.
Poké Wars. The authors sometimes get a little too detailed and graphic with the descriptions of what undampened Pokémon attacks do to living beings..
Cheerilee's Garden. In particular, Sweetie Belle's death, although the others could probably spark some unpleasant reactions as well.
Project Mobitropolis, Shax the Raptor's gestalt of Sonic 2 and the backstory behind the SATAM continuity, has an in-universe moment of nausea induction when Prince Martin is deceived by Robotnik and roboticized and subsequently murdered/deactivated, and it proves to be horrifying enough to make Sonic himself puke in revulsion.
Hivefled; the prequel Reprise was intentional Torture Porn, and while the sequel's not so graphic it does carry over.
The Prayer Warriors repeatedly point out that they don't bury the enemy's dead. One has to imagine all those bodies, laying around rotting and getting eaten by scavengers. Some of the death scenes also count.
Virtually any time someone is stoned to death, especially if you're familiar with how it works in Real Life, as it's a long and painful death.
Chiron is killed and eaten by the residents of Camp Half-Blood.