In truth, a lot of fairy tales are already dark to begin with. (Well, the original versions)
Red riding hood ends with her and grandma dead, Rumpelstiltskin ends up torn in half, The evil queen in Snow White was going to eat Snow's organs, but in the end ends having to wear hot iron shoes and is made to dance until she dies, the list goes on and on
The Grimm Brothers version of everything is morbid. I liked the darker version of Hansel and Gretel I read in a fantasy magazine, though. In that version the with teaches Gretel to be a witch. Yes, Hansel does cook the witch, but now Gretel is a witch who will come back to the cottage someday.