Per KOTOR, Ajunta Pall's spirit becomes remorseful of what he had done and was redeemed with Revan's assistance. I don't remember how it was worded in the Darth Plagueis novel, but it was something to the effect that the Jedi needed an excuse to "fall" to the Dark Side and in that way make excuses, while those like Palpatine and Plagueis were submerged in the Dark Side from the beginning. It was elaborated on more than that, but I don't have it in front of me. Someone like Dooku fell to the Dark Side for political ideals, and Vader fell to save his wife. His excuse for falling was convincing himself that it would save Padme, which was undone when he realized that he had to abandon the Dark Side to save Luke. I do not think Plagueis' point was that a fallen Jedi couldn't be evil (Darth Vader did very horrendous things) but that they required a justification to fall that could be undone. Ajunta Pall was redeemed posthumously and he was one of the first. With someone like Palpatine, redemption seemed like it just was not going to ever happen because he was trained in the Dark Side pretty much as a way of life vs. something he turns to or has to justify. Though the novel itself also establishes that there was a Sith of the Bane line that was redeemed to the Light Side. So, unless that Sith turned out to be a fallen Jedi, he would prove that there are exceptions. It's impossible to know, but if Maul did not die (which would have involved killing Obi-Wan), then Anakin would never have become a Jedi most likely. Maul would have stayed in the picture. Dooku may or may not have been manipulated to still serve as leader of the CIS. And ultimately Sidious would still have to worry about his apprentice turning on him. But unlike Vader turning on him to redeem himself and save his son, Maul would probably turn on him in an effort to usurp him as Sith master. Or conversely, with the battle droid army shut down in TPM and the pilots returning from successfully destroying the droid control ship, Maul might have pulled out despite his victory over the Jedi to evade the Gungan army. Anakin still might come to Palpatine's attention, but he might be able to snatch him up before the Jedi (who already seemed to reject him) and Anakin could have been raised as a Sith and ultimately replace Maul at a later date. But ironically, though Maul might be the weakest Dark Side out of himself, Dooku and Vader, it seems like (IMO) that he would have been the safest bet for preserving the Sith.