Grimm's Fairy Tales are already watered downed versions of the originals, edited for the benefit of softer 19th century sensibilities.
Darker material gives stories gravitas as well as putting the moral lessons in starker, simpler terms. If you don't follow instructions, or exhibit some antisocial behavior, you're not just "punished", you're slaughtered. Little Red Riding Hood strayed from the path and was eaten alive by a wolf. You can't get more clear than that. If and when the hunter saves her, that's mostly for the benefit of the parent, not the kid.
I'll think you'll find that most children, even from a very early age (preschool and before), are quite capable of discerning fiction from reality, and adept at distinguishing and recognizing (if not understanding) the moral lessons. The problem, IMO, with many fairy tales isn't the gore but that neither the moral lesson nor the plots resonant; so the gore just seems gratuitous rather than punctuation. Nobody is afraid of wolves anymore (we killed them all generations ago), and we rarely if ever let kids out into the world without safety rails, even up to college. The latter wouldn't matter if kids feared wolves--the graphic detail would make the moral lesson stick, which is kinda the point--but they just aren't.
I think you'll also find that, especially in the U.S., we live in a ridiculously violent culture; violence that we at best only superficially disguise. It's not the guns dropping legions of "bad guys"--again, most children understand fiction--it's just... I dunno, violence as an end unto itself rather than a means...? Fairy tale gore in books is the last thing I would ever be concerned about. I've actively tried to read these sorts of stories to my kids, to add diversity to their diet, but they don't seem to ever get into it. I think maybe it's just too unbelievable and dull, not too raw. The modern world is too different, modern culture much more slick and appetizing, and they know it even at 3.