I didn't really find Ultimecia undeveloped at all. When possessing Edea in Galbadia in Disc One, she basically says (something to the effect) that Sorceresses have been hunted down and persecuted for generations. We even see that the SeeDs were hunting her down across generations, and now even freaking time itself. She's fed up with it, and is going to take extreme action via Deus ex Machina 'liedk omg nihilism!1one'. Granted, not the most detailed villain, but there's more to her than meets the eye.