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: 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: When viewers are introduced to Jack Torrance, he's a devoted husband and father. He takes a job at the Overlook Hotel so he can support his family while also putting aside time to work on his novel. As he soon finds out, the dark powers that possess the Overlook have different plans for Jack.
Gone Bad: The Overlook slowly takes control of Jack. At first, its influence manifests as nightmares and outbursts - then things get much more dire. After encountering some of the spirits that haunt the hotel, Jack turns on his family and hunts his wife and son with an ax.
The Good Guy: Sweeney Todd is the bad guy incarnation of Benjamin Barker. Barker was a loving husband to his wife Lucy up until an evil judge decided to claim Lucy for himself. Judge Turpin convicted Barker of a crime he didn't commit and banished him from England. Turpin then preyed upon Lucy, who took poison to end her life in a fit of sorrow.
Gone Bad: Barker returns from exile as Todd. He sets up a barber shop over Mrs. Lovett's meat-pie shop. After that, he proceeds to exact a bloody vengeance on practically anyone who comes to him for a haircut or shave, with his ultimate sights set on Judge Turpin. While his anger is understandable, by the end of the story it's hard to justify Todd's actions.