Since we've been discussing Star Wars, with some peripheral comments about Darth Vader, now might be a good occasion to discuss the theme of how some people go from hero to villain. That's an interesting question on several levels, whether in fiction, theology, or real life.
I always thought of Darth Vader as a cartoon villain, so I can't bring myself to take the fall and redemption of Vader seriously, even at a fictional level. But let's explore some better examples.
In Scripture, the two paradigm examples are the fall of Lucifer and the fall of Adam. What Scripture says about Lucifer's fall is oblique and shadowy. Although Scripture is more explicit about Adam's fall, it doesn't say what motivated him. In that regard, Lucifer and Adam are ciphers. But one thing they both represent is apostasy.
A more detailed example is the rise and fall of King Saul. At the outset he's engagingly bashful and modest. The reluctant hero. Doesn't wish to be king. He seems to be a conventionally good young man.
To the extent that he has a "tragic flaw," he's not obedient. If something makes sense to him, that's what he does; if something doesn't make sense to him, he ignores it. It never registers with him that he has a duty to obey God just because God said it.
But having been loath to be king, he becomes very possessive of his position, to the point of murderous resentment and envy when David outshines him in popular acclaim.
An aggravating factor is his sense of betrayal. He feels his son betrayed him by siding with David. Although his reaction is psychologically understandable, it doesn't occur to him that Jonathan finds himself in a dilemma. All things being equal, Jonathan ought to be loyal to his father, but if his father is in the wrong, and David is innocent, then what is Jonathan supposed to do?
In his fanatical effort to cling to the throne, Saul alienates the people closest to him. David and Jonathan have no intention to commit mutiny; rather, Saul is the one who pushes them away. In his paranoid fear of sedition, he creates the very thing he fears.
Likewise, he murders the priests who gave David asylum, on the precipitous assumption that they, too, must be traitors and coconspirators.
Saul is in a moral freefall, where everything he does just makes the situation worse, because everything he does is worse. He brings about his own destruction by trying to save his throne, whatever it takes. It's a frightening example of how someone can lose all perspective.
Sometimes a sense of disillusionment can be the trigger. I once saw an interview with Gen. Jaruzelski, one-time military dictator of Poland. He was, of course, an atheist. You don't get to be the head of a communist state, itself a puppet gov't of the old Soviet Empire, unless you are a card-carrying atheist.
Yet in his younth he was a devout Catholic. Like other pious Polish Catholics of his generation, he was brought up believing that the Black Madonna of Częstochowa was a talisman that would keep the invaders at bay. As long as Poland had that icon, it would ward off her enemies. When Russia invaded Poland, his faith was shattered.
Another trajectory is crossing a line of no return. They may be initially hesitate or even agonize, because they believe that if they step over that line, they can't turn around. Once they take that fateful step, that commits them to everything else which goes along with their momentous decision. The door locks behind them, and they find themselves in a small, dark, windowless room with no way out.
In a horrific way, this can be "liberating." Have done something unforgivable, they no longer have anything to lose. Every additional atrocity is par for the course. The first atrocity was the game-changer. If they are irredeemable, then there's absolutely nothing left they aren't prepared to do.
I've read Jews who complain that Christianity represents cheap grace. Just come to Jesus and all is forgiven. Makes it way too easy to get off the hook.
Without taking time to respond to that directly (that's an argument for another day), what this objection overlooks is the alternative. If a person has no hope of redemption, then there's nothing left to deter him. He might as well be bad as bad can be. He has nowhere to go but down. Despair is a recipe for sociopathology.