You bring up a good point about Ultimecia. She never really did anything that evil, or at least that directly effected you and your party. You, the player, had reason to hate Sephiroth because (SPOILER)he killed Aeris. You had reason to hate Kefka because (SPOILER)he killed HUNDREDS of people mercilessly, including the newly reformed General Leo. What did Ultimecia do to make you have desire to kill her? Sure, she (SPOILER)possessed Rinoa, but she turned out fine, and she even gained some nifty new powers afterwards. Not to mention her being possessed brought her and Squall closer together. Dammit, the party should have been thanking her at the end, not fighting her.

man spoiler tags can be annoying sometimes...