During the period of mass-Christian semi-conversion managed by Olafr Tryggvason, Icelandic saga writers borrowed from Celtic folklore the meme of the super-powered female antagonist. But this was not a stock Nordic character, and so the Icelandic writers turned these Celtic villains into trolls.
Reading Norse folks tales both old and new, it’s not totally clear to me that trolls, in the materialist Nordic forest cultures, aren’t a disrespectful way to depict people, within Nordic society. exhibiting antisocial qualities and brutishness, from either elite or hoi poloi perspective. Grendl and his mother, for example, were trying to beat back and protect themselves from a community of vicious marauders. Trolls aren’t really outsiders. They’re big, ugly, thuggish, irresponsible and dangerous people who live next to and interact with the human or protagonists a lot. Often trolls are distinguished by long noses, for example, a trait that tends to crop up in Scandinavian people anyway. Modern author Rolf Lidberg’s trolls are clearly just common folk. And trolls and people tend to marry or morph into each other over the course of some stories. After all, troll literally just means magic. Protagonists, including when they seem very much to be representing the youngest member of a family audience, are depicted as underappreciated princesses and princes, while in some stories, characters that start out as talking animals end up being princes. So there’s a lot of fluidity there, between behaviour and metaphorical nature, and there’s a clear tendency to hyperbolic representation in these folktales.
Usually the mighty Icelandic troll ladies were slain by the saga hero, though sometimes they were spared in exchange for treasure, or befriended, as in the case of Brana & Halfdan, according to Martin Puhvel (1987, McGill).
Normal but vengeful females played the boss villain role in Celtic folklore, and usually it took the help of an animal, such as a dog, for the male hero to defeat them. Cats were however associated with the abjected feminine and villainous hags, the cailleach (Puhvel 1987).
As Silvia Federici documented (1998), the crippling fear of women is often the result of instrumental, imperial-cosmopolitan divide-and-conquer interventions imposed upon hinterlands communities. Eleanor Hull’s research (1927) showed that female Celtic supernatural villains like banshees and various evil, watery tarts were degenerate descendants of ancient, mighty war goddesses like Macha and Bodb.