They could do it very easily basing it off the large body of works, not a specific series.
Arkham Asylum is a fantastic game, It's dark, it's gritty - it's also only rated T.
The same thing could very easily be done with a movie. Personally I think Carnage would work being PG13 perfectly, a character isn't validated by how much blood or gore he can spill on screen.

This personally goes back to an old Ghost Rider comic book, where a scarecrow esque characters makes his first appearance. He takes a mothers child and kills it, then her. You don't actually see any of it - but it's still terrifying.