30 "H"s/WMG

The music Harry plays when he "rocks the fuck out" is Level 13 on Mohs Scale of Rock and Metal Hardness
The Mohs Scale of Rock and Metal Hardness page states that "Anything beyond 11 = Noise." This implies that there is, indeed, a Level 12, but none of the music noise that would be catagorized as such can be listed. What Harry plays on Fuckslayer is above this...so impossibly extreme that it manages to warp the universe even more, which is why the story gets even crazier after the second chapter. Throughout the story, music with hardness levels as high as 14 and even 15 are played, which is now possible because the limit to music hardness has been broken.
 * Alternatively, Harry is either Karl Logan or Irrumator

This story was actually written by one or more of the members of Dethklok, under an alias
Probably not one of the Scandinavians, unless it was a collaboration, since the author seems to have a better grasp of English than Skwis and Toki do.
 * Given Skwis has expressed interest in Harry Potter before, he may have written this and then had Charles find a translator/editor for him. Perhaps he's Eloquent in His Native Tongue.

This story is true.

 * If it is, my life now has meaning.

Thirty Hs takes place in Classical Mythology
When Harry "kills the fuck out of" Mercury and Venus, he is actually killing the gods Mercury and Venus. The sudden death of the goddess of beauty goes a long way towards explaining why every vagina in the galaxy explodes after that event.

This story can defeat Cthulhu
The insanity caused by this story's plot is so awesome, and so mind-warping, not even Cthulhu can face it. On a related note...

The book Harry's reading is the Necronomicon
Admit it, a book with pages made of lasers and letters made of headless women making godless love with dragons made of motorcycles would drive all but the unworthy mad. The author knows that's what the Necronomicon's like, because he read it shortly before writing this fanfic.

Rape Radbury is Slender Man
He's got the height, the lack of a face, the suit and the ability to appear out of nowhere. It's entirely possible Harry is capable of having reasonable conversations with Slendy because Harry simply can't become any more crazy. Additionally, the "critically acclaimed fiction that always turns into fact" is in fact his mythos, which originally he published as a series of novels before Self-Fulfilling Prophecy took over.

Rape Radbury wrote this fanfic.
Think about it- it says that everything he writes comes true, no questions asked. So if he wanted to write complete craziness, it'd still happen.

The story nearly triggered Spiral Nemesis.
That's the reason it ends abruptly after Harry spits the thumbwater at the old man. It got so mindbreakingly awesome, the universe couldn't handle it continuing. Either that, or Anti-Spirals just killed the author.

The Old Man who removes his head is inducting Harry into full godhood.
Harry has become such an obscenely powerful Physical God that he is captured by a mysterious Inquisition, and is then taken into a deep and fiery Hell to test his true physical strength. Upon emerging from the Hell, he is given a choice of creature that will undyingly serve him. Given Harry's nature, all the creatures are horrific mutants. The Old Man is the mighty god and creator of the universe, or else lives on an entirely separate plane of existence.

Fuckslayer has a mind of it's own
And it's controlling all Harry's actions.

Wizard People, Dear Reader is the prequel to Thirty H's
Which would make Harry's attitude in the former much more explicable, as well as his godlike power.

The story is set in the Warhammer 40,000 universe.
Because It is the only possible setting matching this fanfic level of violence and sheer audacity. Harry and Dobby even sport a skull adorned space armor with GROINSAWS ATTACHED TO THEM.
 * Continuing on this, maybe it was an Angry Marine's attempt at Fan Fiction? It would explain the groinsaws and "killing the fuck out of" things.

Fuckslayer contains The Music from Soul Music
Just as an above WMG suggests, maybe Fuckslayer is in control. Just like The Music from Soul Music. It overpowered Harry, but mutated, and thus making him evil.