Seifer needed more character development. I don't believe he was just an asshat at the orphanage because he felt like it. There was something making him insecure. Perhaps Squall's crybaby attitude got him more attention from Edea, or maybe it was just that Seifer wanted more time with Ellone and Squall hogged her. Who knows? But I do agree with the general sentiment that Seifer is an excellent character, but he could have and should have been even better.
Seifer is utterly detestable for the most part, and in that he's well written, though his general childishness frankly gets on my nerves. However, I really don't understand his motivation in the game. We know he was bewitched by Ultimecia early on, but I'm not sure what that precisely accounts for. Other than that, he probably keeps following her because he's too arrogant to admit his mistake and slowly sinking into madness. However, this is badly brought into the story, and just as badly conveyed through his dialogue. Most of the stuff he says in mid-to-late game comes out of absolutely nowhere and doesn't make too much sense.
At the start of the game you can find that Seifer got a book from the library called "The Sorceress and her Knight" or "The Sorceress's Knight" or something; it explains why he is so infatuated with the idea of serving Edea. I think he is a bit of a jerk but that could be born of insecurities or something. He's an interesting character, I just wish he was done a little better
I can only think of him as an antagonist. He did not do anything nearly as drastic as other villains that were a part of a larger plot, such as Golbez, who caused the death of Tellah, or Exdeath, who caused the death of Galuf. If he had killed or caused the death of one of the protagonists, like Quistis or Zell, then maybe he would be seen as more villainous.
I believe in the power of humanity.
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