The horror genre (that includes creepypasta, as that's a subgenre of horror and more akin to urban legends) itself is not bad. Horror stories have existed since Man could weave a tale by firelight. Horror is not everyone's cup of tea, yes, but it in and of itself is not bad and does not spawn serial killers—unless they're fictional.
This is gearing up to become just another witch hunt. Back in the day, comics were to blame. It was a relatively new media that adults at the time were not very knowledgeable of. Thus, you had stories of kids jumping out of windows, because they thought they could fly like Superman. You had stories of kids committing violent crimes because they read EC Comics' fare like Crime SuspenStories, Weird Science, and The Vault of Horror. Dr. Fredric Wertham became a household name with his book analyzing this supposed crime-comic correlation, Seduction of the Innocent.
Flash-forward to the early 90s. Doom has just hit PCs. It's a gory demonic mess that's enjoyed by young and old. Then comes the Columbine school shooting and the perpetrators had made Doom level mods (which weren't that great, by the way. They're still available to play, and they're poorly-done). Suddenly, Doom is bad.
Early-Naughties rolls around. You suddenly start seeing cases which blame Grand Theft Auto. GTA and similar games are now the target of parental watchdog groups everywhere. Do you know what your kids are playing?
Slenderman and creepypasta is just next in line to be the next media scapegoat, mainly because once again, parents are not providing proper guidance, are not knowledgeable about the material, and are quick to pin the blame onto something that they don't understand.
For the record, Dr. Wertham became an advocate for the comic industry in the 70s, and having read part of his book, he actually researched comics. Whether or not you agree wi his arguments, you could see the education and research behind his conclusions.