What is the scariest horror thing that could happen to Spiderman?

A Monty Python-esque giant boot that comes out of nowhere & squishes him... It's the only thing to do to a spider ;) 
 
I think in terms of 'horror' then there's a lot, but it would be gruesome.  Horrible stuff that involve mutilation, usually.  I don't tend to think of Spider-Man as a 'horror' type of comic (although the amount of characters who've come back from the dead might count as zombies). 
 
For Spidey, if he was powerless & therefore unable to protect his friends & family then that would be a giant nightmare for him.  That said, I think having the perfect life in the House of M... and then having all that stolen from him... and remembering it.  Yeah, that's got to be a horrific thing for anyone to live through.

To Peter it would be to see his loved ones hurt because he decided to be selfish. Y know throw his costume in the garbage one to many times and have Carnage find the suit on and then go on a killing spree new York has never seen before while in his suit. Peter's worst nightmare I think 
 
As a reader I think to witness Spidey just get a horrific beat down with no one coming to help him. to wek to fight  stronger opponent and nothing he can do about it 
poor spidey 

His comic gets canceled! O_O

Realizing how stupid his decision to save aunt may was

Norman Osborn just after waking up! 
lol 

"

What is the scariest horror thing that could happen to Spiderman?

"

Aunt May becomes a vampire, and tries to bite him.

Spiderman mutates into a giant man-spider that shoots webs from his butt and then eats his loved ones (i.e. Aunt May).

Aunt May dies. 

" Joe Quesada coming up with another idea to 'fix' him. "

" @Mr.Q said:

" Joe Quesada coming up with another idea to 'fix' him. "

agreed, OMD was already horrifying enough for me. "

" @Mr.Q: Peter Fonda? the mephistopheles guy in ghost rider? "

  The scariest thing that can happen to Spider-Man is him growing up.  Just ask Joe Quesada, he would agree.

Having to fight an enemy he can't beat I.E Morlun. It already happened though...

Entering a Superman crossover. 
Oh yeah having a cartoon show on fox too!

i think poor zombie spiderman had it worst.  he woke up one day out of his disease to find hed killed mary jane and aunt may--and then eaten them!!  after that he became a hero for a while, but then sandman killed him because he mistook him for a villain.  :(

Being just plain old Peter Parker, with no powers and no Spiderman.   The fact that Spiderman never existed would scare the shit out of me. 

" @Lustwish: If Spider-Man never existed, comic books probably wouldn't be as popular as they are today. "

Spider-man Unlimited.....

" Having to fight an enemy he can't beat I.E Morlun. It already happened though... "

                                                                                                   To become SpiderpIg

His worst nightmare is of course having another Uncle Ben or Gwen Stacey moment; loosing someone he loves because of something he did or didn't do. That's why he is Spider-Man after all.  
 
But I love some of the other suggestions in this thread. XD

I think it'd be if Joe Q. decided to have Mary Jane marry Venom.

It has already happened.
SM3.

Is it true he is going to die?

It's a tie between the Green Goblin killing Gwen Stacy, and his fight with Morlun.

I think his worst nightmare might be something that's already happened, which is Back in Black, having his Aunt May murdered. But what would be even scarier would probably be someone who hunts not only Spider-Man down after learning his secret identity, but everyone he loves, torturing them, one by one, slowly, before his eyes, until he is finally shot in front of them (his loved ones aren't murdered yet, just tortured). Then the loved ones are shot.
 
Yep. I'm a sadist.

If Eddie Brock had killed Aunt May in "The Last Temptation Of Eddie Brock", with Peter in the building at the time, I imagine that would have been an incredible guilt trip to go through. Especially since he wasn't in her room, no, he was busy with Madame Web trying to think of stupid ways to bring her back, when he should have been spending her last few moments together.