Calvin & Hobbes. Rule 34, I know that one of your taglines is about ruining childhood. I'm also well aware that this already exists, but for the love of bite me, please, PLEASE be gentle with this one! Putting aside the nostalgia, as well as how legendarily creepy the rule 34 would look (gay pedo doll/furry, none of these words should be even seen in ANY combination), anyone that can successfully (cough) enjoy this would, to me, be of seriously unsound mind and soul.
My Little Pony: Friendship is Magic. Yes, I know this one also exists, and since it attains a near-cult status, how could it not? That doesn't mean I can't wish that it DOESN'T exist. Like Calvin & Hobbes, I can't help but feel this show should be held above the usual filth. Romantic shipping is one thing. The inception of "Princess Molestia" is another but still tolerable. When you bring rule 34 to its illogical extreme, however, we get things that, again, like Calvin & Hobbes, prove that there are some people out there who need to be on a lot of drugs, and in a straight-jacket.
You know what? I'll just say ANY rule 34 that involves snuff and gore. Hell Season is a great place to start. ANYTHING that invokes memories of Hell Season should not exist, no matter what the source material may be. I would not wish those levels of gore and ultra-violence on anybody or anything.
Incidentally, if you don't know what Hell Season is, I beg you, I plead with you, I would bribe you if I could, NOT to go looking for it. I used to think I was made a stronger person for having survived it, but I now realize it has only made me a slightly emptier, colder person.