Eh...Seifer isn't really the type of character you expect to drive the plot. He's basically just there most of the time to provide a rival for Squall, and he does do a pretty damn good job of that, to say the least. The two supplement each other well in terms of ability and serve as stark contrasts to one another when it comes to motivation/personality. That type of character rarely has a huge impact on the plot. In fact, it's fairly common for the main character's rival to serve as second banana to the real major antagonist. They're not exactly famous for being catalysts, just...rivals. They exist to impede the protagonist. There are exceptions to this rule, but they don't tend to take place in JRPGs.

Seifer is, morally speaking, a bit hard to peg following a nonconventional school of thought. If you want to be simplistic about it, then yes, he was a villain...or at least an anti-villain. He endangered the lives of many people and even offered Rinoa up to Adel at one point. It's fairly easy to classify him as a genuine villain, but looking a bit deeper into his character, he was just trying to achieve his life's dream. As said, his "romantic dream" was all the poor guy had to cling to at that point. He'd been stripped of pretty much everything else; he'd lost his parents, lost his friends from the orphanage, failed to make it in to SeeD again...Life was really screwing Seifer over.

I guess it all comes down to Seifer being a victim of circumstances. It's not his fault he went insane, it just happened because so much crap went on in his life that he just couldn't keep it together. The guy might've consciously realized what he was doing, but at that point, he wasn't thinking clearly enough to care. Had he been in his right mind, I sincerely think that Seifer would've turned back at some point. You just have to understand the magnitude of what was going on in his life to really understand his character.

Yes, but look at what he'd gone through prior to that. Somehow, he lost both parents at an early age, was stripped from his beloved orphanage so that he could be taught how to kill people for a living, then failed to make it as a member of that very organization several times. All he had left to turn to fall back on were his impulsiveness, his skill in combat, and his "romantic dream." The guy had genuine problems. You couldn't have expected him to be thinking very clearly at that point. Yes, he wanted to be doing something big, but had he been in his right mind, I really don't think he would've gone to such great lengths in order to accomplish his goal.

Squall even said that Seifer was one of them. Again, they were cut from the same cloth, and had similar goals in mind at the beginning of the game. They were simply going about it in different ways. Once Edea came into the picture, things became more complicated. Seifer, after being put through the ringer and having everything stripped away from him, was offerred a chance to actually make something of himself and realize his dream. He had nothing else to do and his mental stability was beginning to wear itself down. The guy really didn't know what he was doing.

I don't think that Seifer should be completely forgiven, though. He did some horrible things, but his perception of right and wrong were probably horribly skewered and warped by that point considering just how messed up in the head he was. He'd been orphaned, abandoned, deemed a failure by the Garden, was made a social outcast...There's only so much the human mind can take before it breaks down. That's the only reason why I don't consider him a villain.