For me it comes down to, what breaks a good man? Anakin Skywalker has seen a devastating war which has ended the lives of his friends, left a comfortable life which caused him scorn from his family, watched a corrupt senate bicker while civilian lives are at stake, fought a manic enemy determined to dismember the Republic, and all the while he holds back. Tells himself not become aggressive, to show patience for his allies, mercy for his friends, to quiet that voice in his head telling him that so many problems could be solved if Jedi would just take charge of The Republic.

By Episode III he is just barely holding on. They're winning the war and on the road to peace, but so many fundamental flaws in the things around him have Anakin's mind clouded with doubt. And when one of the last remaining Mandalorian forces seemingly kills his wife and child? Something snaps and all the the things he's bottled up, the disdain for those in the Republic not appreciative of the military's sacrifices, the cruelty towards the enemy he should have displayed, his resentment towards Obi-Wan for being more focused on being a leader to the Jedi and his self congratulatory pride at being such a good teacher to Anakin; all of that comes out.

And the guilty self loathing plays a part too. All his abilities, hours of training, countless dedication, and he still couldn't save the people he loved. Anakin Skywalker can only see failure within himself and that's where Palpatine makes his move. The man who tells Skywalker that all his suppressed instincts are in fact right, that they can build a better Republic, that their enemies will not be allowed a merciful surrender, that they can create a galaxy where no innocent people are allowed to be killed because disorderly scum were allowed to roam. That is the instant where Darth Vader emerges to offer Skywalker a new, better path. And the moment when the young Jedi allows himself to justify the annihilation of the Jedi is the moment where Vader essentially betrays him.