Red Superfly
You creepy little stalker
Well now you have put it like that, Carnage would make a great bad guy - especially if it was in the hands of Sam Raimi.
However, Carnage really is part of Spider-Man's "truly sinister" line-up of villains.
I even think they'd be pushing it with Venom - especially if they want to do him right. The idea behhind Venom is to create the ultimate nightmare of Peter Parker - this means he would have to be pretty scary. How could you do this when the censors almost had an anneurism over the Green Goblin two years ago?
So, considerring that, Carnage is like Venom's-evil times two. There's no way you could get away with Carnage. That's why I don't ever think Spider-Man will ever face Venom or Carnage, because there's many other less frightening villains to use - some of which are more famous and loved that either V or C.
A Carnage slasher flick could be what the teen horror industry needs to turn itself into something thats taken seriously. You could make one heckuva scary movie out of Carnage. It'd be a cross between Alien, Predator, and Halloween - except scary.