There is a darker side to the Aarne-Thompson folktale type 300 – Dragon Slayer. This is when the Princess actively betrays her doomed family. For example; the Hero Theseus and other youths were sent to Crete as human sacrifice for the Minotaur. The Minoan Princess Ariadne fell in love with him, betrayed her country, helped him defeat the national monster (which happened to be her half-brother) and they eloped. In route home to Athens he abandoned her on the island of Naxos. The god Dionysus (her second cousin) just happen to be passing by heard her lament, fell in love, made her his wife, and raised her among the immortals Olympus. (Pausanias, Description of Greece 1. 20. 3)
As I pondered this variation on AT-300, Medea came to mind. She was the Colchian princess who betrayed her country, helped Jason (and the Argonauts) defeat the dragon that guarded the Golden Fleece. He was born from the blood of Typhon split upon the Earth. Medea and Jason steal the fleece and elope, he abandons her and she ascends to heaven in a winged chariot sent by the sun-god Helios.
Princess Scylla fell in love with King Minos while he assaulted her father’s kingdom. For love of Minos she plucks from her father’s head the sacred lock of purple hair that protects them. Minos abandons her and the gods turn Scylla into a sea-bird. (Metamorphoses, Book 8)
So the darker side of AT-300 would be: A princess betrays her country and helps the hero defeat the male magical element protecting the country. The hero abandons her and the gods intervene on her behalf. Betraying your father doesn’t seem like a really good idea if you are a princess in Ancient Greek mythology.