Wasn't Slender Man just a Something Awful thing for a while? I don't think it was ever a video game. As tragic as the stabbings were, it's folklore, like Bloody Mary or Beetlejuice.
It's an internet meme that grew into an urban legend-style horror story, mostly thanks to the "Marble Hornets" YouTube videos.
Also, all Beck's research tells him is "it's a video game." Someone told him about "Slender," and he's swapping out the game "Doom" from the age-old script about what's wrong with kids these days.