Big-Lipped Alligator Moment: the Gingerbread Man. It climbs up out of a well, kidnaps a little girl, and jumps back down the well just as quickly as it appeared. If it didn't explicitly say that it was the Gingerbread Man, you'd probably never have known thanks to the utterly horrificand disturbing way it's presented, and unlike most of the other fairy tale-based elements and characters, it never appears again.
Complete Monster: The Mirror Queen was once a beautiful and vain queen who left her people to die of the plague; prior to this, the queen tortured and murdered numerous people for Black Magic to grant her immortality. Failing to get eternal youth, she spent centuries rotting away from age and the plague before brainwashing an innocent huntsman into becoming her werewolf slave, and began kidnapping young girls via macabre versions of fairy tales, burying them alive to take their blood for her youth and kills the soldiers investigating these kidnappings. When one child is saved, the Mirror Queen takes the huntsman's daughter Angelika as a substitute. When the Grimm brothers confront the queen, she magically forces the brothers into a knife fight, brainwashes a mortally wounded Wil, and mocks Jakob about his failure to save his sister as a child.
"Hansel and Gretel" (KMH 15): The Wicked Witch is the archetype for the evil witch in the woods. Described as wicked and godless, she lays a trap with her edible house for children, whereupon she captures them, kills them, butchers and cooks them for her feasts. Upon capturing Hansel and Gretel, she attempts to fatten up Hansel, before growing tired of his seeming inability to gain weight. Intending on burning Gretel alive before eating her brother, the witch proves to be one of the most terrifying and evil monsters in any of the Grimm brothers' stories.
"The Robber Bridegroom" (KMH 040): The Robber Bridegroom himself appears to be a charming suitor seeking the hand of the heroine in marriage. In truth, the Bridegroom is the head of a group of bandits and a cannibalistic Serial Killer who tricks the women into coming to his home whereupon they are murdered and carved up for food. When the heroine visits and hides, she witnesses a young captive dragged in and killed by her fiancé and his men, realizing the fate that would await her, with the Robber Bridegroom being a dark reminder that evil may hide beneath kind and trusted faces.
"The Juniper Tree" (KMH 047): The evil stepmother, upon marrying the husband, grew to resent her stepson, knowing that he would inherit the family's wealth when he got older. Forming a plan, she convinces her stepson into looking into a chest for an apple, only to then decapitate him by slamming the lid onto his neck. The stepmother uses a bandage to reattach her stepson's head, and she manipulates her daughter, Marlinchen (or Marlene in some versions), into thinking that she killed her own brother. She then takes the body, cooks it into a stew, and she serves it to her unsuspecting husband. The story repeatedly makes it clear that the stepmother cared only for herself, and that she committed these actions out of greed, as well as a genuine hatred for her stepson.