The_Oathkeeper
Why so serious?
Plus, Seifer has some advantages on Squall. He's a far fiercer fighter, he can use magic without GFs, etc..
Storyline-wise, I'm actually not sure that GF's are required to use Para-Magic at all. Of course I realize that, as far as gameplay is concerned, no one can use standard Magic (under normal circumstances) without a GF Junctioned. However, as far as the storyline is concerned, Para-Magic and Guardian Forces are typically referred to as separate from one-another. Though I can't remember the exact quote, I believe a tutorial in the game said that what makes Balamb Garden stand out among the other Gardens is its use of Para-Magic and Guardian Forces.
Then there's the fact that Seifer's ability to use Magic without a GF Junctioned is never mentioned in the story. Don't you think that would hold some significance in the storyline if only certain people could use Para-Magic without a Guardian Force Junctioned? Plus, if I remember correctly, I seem to remember SeeD Trainees using Magic during the Garden Wars. Of course, I don't know how much weight this holds, considering SeeD are trained in the usage of GF's from pretty much the moment they arrive at the Garden. I just think that it's unlikely that Seifer is a special case. Besides, doesn't he only ever use Fire Spells?
I'd say that GF's are neccessary in order to use stronger Magic, but, storyline-wise, most of the characters can probably use the weaker Para-Magic spells without aid from a GF. Again, I realize that this contradicts what we see in gameplay, but the storyline does refer to GF's and Para-Magic as separate abilities. Plus, I really think that, if Seifer were the only student at Balamb who could use Magic naturally, they would've pointed that out at some time during the storyline. We could always just chalk it up to an error on the part of the creators, though.
As for Seifer's insanity...Well, maybe he wasn't batshit crazy just yet, but he definitely wasn't sane enough to think clearly and fully comprehend the consequences of his actions. Yes, he knows what he's doing, but he doesn't understand the impact his actions are having on the world around him. He'd just been through so much that he finally cracked. It's not as obvious as some characters with emotional issues (like, say, Sephiroth during the Nibelheim incident), but Seifer definitely has problems.