Gruesome imagery, as the title might imply.
One of the largest categories of the Creepypasta franchise is Lost Episodes, supposed episodes of shows that never aired due to being too morbid or dark for the audience to handle. Some popular one's include Suicide Mouse (Mickey Mouse), Dead Bart (The Simpsons) and the entire "series" of Candle Cove. Squidward's Suicide, however, takes the cake.
The story comes from the perspective of an intern working for Nickelodeon who views a disturbing "lost" video of the show Spongebob Squarpants. The episode depicts Squidward putting on a clarinet recital and being booed off stage, even by Spongebob. The entire audience has hyper-realistic, bloodshot eyes, and they laugh at him until he leaves.
The rest of the episode consists of Squidward sobbing uncontrollably and is interrupted a couple of times by photographs of disemboweled children (told you it was disturbing). Squidward's eyes turn the same hyper-realistic red that the audience had previously. He ultimately ends his life by putting a shotgun in his mouth and blowing his brains out, after a voice says "Do it!".
What makes this unnerving is that, like I mentioned in Lost Silver, it's taken straight from something near and dear to childhood Matt (plus, Squidward was always my favorite character anyway). I find it disturbing in a way that most other entries on this list can't even compare to. Relatively short read, and I think it may have been one of my first forays into Creepypastas. Red Mist is just an expansion, and I'm not even sure you can find it anymore.