Change is one of the few commonalities virtually all good fictional characters share. The journey of how they change is what makes them compelling, but not every character makes a change for the better. Somewhere along the way, they discover that deep down they are really villains masquerading as heroes. After this tragic turn, they break bad and eventually become evildoers worse than Walter White.
For those unaware, White is the likable TV antihero from Breaking Bad. He starts the series out as a terminally ill high school chemistry teacher and ends up in a much different place five seasons later. Meek Mr. White gradually transforms into an evil, power-hungry kingpin. The story made for some of the best television of all time, which just goes to show audiences always have a soft spot for good people who turn into villains.
The Good Guy: Michael Corleone is a WWII hero born into a Mafia family. The audience's first glimpse of Michael comes on the day of his sister's wedding - he is a vision of the prodigal son returning home from war. He soon becomes enmeshed in the dirty deeds of his family, and obligation to his people sets him on a collision course with villainy.
Gone Bad: After Michael takes out Police Captain McClusky and Virgil "The Turk" Sollozzo, it's all downhill from there. He goes on to elimiate (either directly or via orders) dozens of people, oversee illicit substance deals, and participate in a variety of other nefarious acts. By the end of the first Godfather movie, he has assumed his father's position in the family.
The Good Guy: Harvey Dent starts out as the District Attorney of Gotham City. He joins forces with Batman and Lieutenant Jim Gordon in an attempt to deter organized crime. On the home front, he is seemingly in love with his girlfriend, Rachel Dawes. But when the Joker puts an end to Rachel and disfigures Dent, all bets are off.
Gone Bad: The Joker compels Dent to seek vengeance for Rachel's untimely demise, pushing Dent to become Two-Face. He mercilessly eliminates scores of people, flipping a coin to decide their fate. Dent eventually takes Batman down with him after Batman decides to carry the blame for Dent's atrocities.
The Good Guy: Anakin is a gifted young warrior with knowledge of The Force. Though his intentions start out in service of a greater good, he is inevitably swayed by the power of evil and makes a switch to the dark side.
Gone Bad: For Anakin, the dark side means becoming Darth Vader. Vader has his way with pretty much anyone he wants to, including children. He gets a (sort of) comeuppance at the end of Episode III when Obi-Wan beats him within an inch of his life. However, Vader survives the incident, is encased in black armor by Emperor Palpatine, and continues to terrorize Luke, Leia, and the rest of the galaxy for several more installments.
The Good Guy: Saruman is a good wizard and the chief of Gandalf's order in the early part of the Lord of the Rings series. Unfortunately, the idea of possessing the One Ring is so overwhelming, Saruman crosses over to the dark side.
Gone Bad: Saruman joins up with Sauron - the Dark Lord of Mordor - to claim the ring and its power. His nefarious acts include imprisoning Gandalf, creating an army to stop the Fellowship, and plotting a scheme to wipe out Rohan.