Glaucus is a sea god, who's famous for his abilities to predict the future. Some even say that he taught Apollo, god of prophecy, all of his tricks. It's kind of ironic then, that Glaucus doesn't see the tragic events of this story coming his way. Why doesn't he know already that Circe's potion will transform Scylla into a horrible monster? Hm, maybe he's so blinded by his love for Scylla that his crystal clear view of the future gets a little foggy.
All of Glaucus' amazing abilities aren't impressive to Scylla, who can't see past his appearance. Even though he's an immortal, prophetic sea god, Scylla can't get over the fact that he's part fish. The beautiful sea nymph just can't see herself with a guy who's blue, scaly, and has the tail of a fish. When Glaucus is transformed into an immortal merman, he may have gained some amazing abilities, but he also unknowingly lost his chance to be with the nymph of his dreams.
The beautiful sea nymph seems to be an innocent victim in this story, and is apparently allergic to wearing clothes. She never asks Glaucus to fall in love with her, and she certainly never asks him to accidentally transform her into an awful monster for the rest of her days. Can she help it that she's just not into dudes who are part fish, even though she is totally in love with the sea?
Some versions of the myth try to make it seem like she deserves this terrible transformation by saying that she's kind of vain. These tellings of the tale say that Scylla likes nothing better than talking crap with her nymph friends about every guy who's into her. These versions of the myth seem to be saying that Scylla is punished for her thinking she's too beautiful for anybody else by being transformed into a hideous monster.
The infamous sea witch, Circe, causes trouble in pretty much every story she shows up in. (Check out Homer's Odyssey for another example.) She's famous for being sneaky and for almost always using her magic for her own selfish purposes. You'd think Glaucus would know better than to go to the sea witch for a love potion. You'd think he'd totally know not to trust her after he rejects her advances. But nope. Glaucus believes that Circe has given him a legit love potion and inadvertently turns Scylla into a monster. Like in so many other myths, Circe proves herself to be untrustworthy, vindictive, and really bad with rejection.