The story introduced Exar Kun near the end of his training; he had mastered the lightsaber, and started poking around in Master Baas's Cabinet of Things Apprentices Do Not Need to See. From this, he learned of the Sith, and their power to destroy stars. He undertook an interplanetary personal research mission, falsely identifying himself as a Jedi archaeologist in order to gain access to Sith artifacts. In time he was contacted by a Sith spirit and led to their graveworld, where he was fatally injured by a collapsing temple. The Sith spirit offered to heal him, if he would only turn to the Dark Side. He did, and that selfish act started him down the dark path. He went on to discover more ancient Sith temples and the survivors of their slave race, and went about exploiting Sith technology and weaponry. By the end he had forgotten why he had started his research into the Sith, he only knew that he had more power than any Jedi alive and that he should use it somehow.
Meanwhile, Ulic Qel-Droma grieved the loss of his master, killed by a Krath ambush. (The Krath were a Sith-inspired cult that ruled a major mining system.) Against everyone's advice, he infiltrated the Krath to learn their secrets and destroy them from within. But once inside he enjoyed the comfort of their lifestyle too much, and twisted by his own use of the Dark Side--and the subtle and gross manipulations of the Krath--he turned on the Jedi and united with Exar Kun. However, until the very last moment, he kept telling himself that he was doing what he was doing to defeat the Krath and avenge his master.
In Star Wars: Dark Empire Luke Skywalker unwittingly retraced many of Ulic Qel-Droma's steps. He infiltrated the Empire's Berchtsgarden and pledged himself to the Dark Side in a ruse to access Imperial military command codes. But--probably due to coercion that takes place off-page--he was cast into utter despair, and became a Vader-like lackey until his sister set him free.
Selfishness, self-delusion, and overconfidence. These compose a Dark Side Trinity more potent than even fear, anger, and aggression. If this model had been used for Anakin, we would've seen him quickly master the Jedi arts and look to anathematized teachings for further knowledge and skill. He would have begun acting like a Sith long before he took a Sith name, or even admitted to himself that he was more aligned with them than the Jedi.
In another universe, several humans were turned to the dark side by the One Ring. The Ring, unlike the Dark Side proper, was a sentient or semi-sentient thing with a will of its own. It didn't offer its bearer any real power, but it did offer possession of itself. This desire to have something was expressed most strongly through Sméagol-Gollum, who gave up the legendarily comfortable trappings of hobbit civilization for centuries of eating fish and orcs in a cave. But as long as he had the Ring he was, if not content, somehow sated. Others fell or nearly fell because they saw in the Ring power, an ability to reorder the world to their liking. Boromir would not have accepted such a pitiful life as Gollum's nor, when he was alive, did he plan the type of despotic regime favored by the Ring's original owner. Doubtless he wanted to claim the Steward's Chair, or even the Throne, and usher in the kind of golden age that he must have believed Numenoreans once enjoyed. Though even the Numenorean state was probably vastly different than the accounts preserved in the Red Book and other writings of the Eldarin and Dunedain civilizations.
Possession for the sake of possessing and an ideal-driven ends-justify-the-means mindset were the road to the dark side here. If this had been applied to Anakin, the Dark Side would have been symbolized in the films by some thing offering great power--maybe a Sith relic, an obscure teaching, or a military command--and Anakin's search for, use, and eventual reliance on this object would drive him into sympathy for the Dark Side. Anakin might've been seeking this goal or object because he believed it was the best way to restore some idealized form of government, or maybe just because it was something grand to possess. Either way, coveting and protecting it would drive consideration for others out of his thoughts.
The last great turn to the dark side I can think of off the top of my head is the Joker's. Sometimes he remembers it one way, sometimes he remembers it another. If there has to be a past, it should be multiple choice. But one way he remembers it is, he had a bad day. He had a bad day and everything went wrong and he saw how cruel, random, and petty the world was. And he got the joke. So he decided to help other people get the joke, too. He doesn't consider acts of cruelty and violence in any way wrong, they're simply the lead-up to the punchline that will free others from the ennui of their lives. Some would say the same thing happened to Batman.
If Anakin had simply been insane, he would've become Darth Vader because that seemed like the best way to accomplish some dubious goal by means of internal logic that is impenetrable to the viewer. For instance, he may have fallen in order to save his wife from dying while giving birth to a child like he saw in a dream.