The feud between the kingdoms is just a bonus. Evil Is Petty: Gothel kidnapped baby Rapunzel for no other reason than out of spite for her ex marrying and having a child with another woman.
And nothing says happy ending quite like being free from Gothel and reuniting with one's family. Earn Your Happy Ending: Nothing earns a happy ending more than being kidnapped and putting up with Gothel for the majority of one's life.
Gothel spends much of the movie proper working to deny Rapunzel any chance of reuniting with her birth parents, indicating that her grudge has not faded with the passing years. She kidnapped the infant Rapunzel in retaliation, rationalizing that Rapunzel should have been her daughter anyway.
In revenge, she decided to kidnap his daughter, frame an innocent king for it and start a war between the two kingdoms in the hopes they'll destroy each other.
She certainly never intended to start a war either. Also, in the original story, Gothel got Rapunzel out of (admittedly rather questionable) deal with her father, rather than kidnapping her out of petty revenge. Adaptational Villainy: Gothel wasn't an ideal mother in the original story, but she wasn't an emotionally abusive slave-driver to Rapunzel like she is here.She's also able to keep three men armed with swords on the defensive in the climax, handily demonstrating that she can take them out with magic faster than they can attack. Gothel here is able to use her magic to create the tower in the first place and imprison someone inside it forever. Adaptational Badass: The witch in the original tale never displayed much magic.Acrophobic Bird: Penelope, still being a young dragon, is a little rusty at very high heights.She's not even so lenient as to let Rapunzel address her as anything but "milady." Abusive Parents: Gothel fits the emotionally abusive type to a T.