It's arguable how much of this is heel-face turn and how much of it is a hidden Face awaiting the moment when his turn will do the greatest good. However it qualifies under the criteria that the audience isn't sure what side he's on until he makes his stand.
Subverted in Terminator 2: Judgment Day, the reprogrammed T-800 Model 101 isn't the same terminator from the first film. Double Subverted in that it was probably also programmed to kill humans before John Connor captured it, though.
In the Hairspray musical, both Velma and Amber Von Tussle are redeemed through the power of THE BEAT, but in the film adaptation, only Amber seems to have one; accepting her loss and walking away from her mother to talk and dance with one of the black dancers.
The Two of a Kind, designed as the comeback film for the Olivia Newton-John / John Travolta duo, features probably the single most jarring and unbelievable Heel Face Turn in the history of film: Satan himself!
The Operative undergoes one of these at the end of Serenity when he discovers the truth behind the secret of Miranda, and just what the utopia he has been fighting for all this time would really look like.
A slow, detailed one of these is roughly the entire plot of the German film The Lives of Others (Das Leben der Anderen).
Zangief in the Street Fighter movie is mainly the Plucky Comic Relief of everyone in Shadaloo, but after Dee Jay informs him that Bison is "the bad guy", he helps the heroes escape. Zangief and Dee Jay weren't evil in the games.
In Inglourious Basterds,the main villain Hans Landa discovers the Basterds' plot to kill Hitler and decides to help them, striking a deal with the Allies and generously demanding medals for the Basterds' bravery. This is all subverted by the fact that Landa is acting purely on whim and self-interest, and is still a total slimeball. Ultimately Aldo is well aware of this, and treats him accordingly.
In Pineapple Express, Red first tries to help the villains kill the heroes, but ultimately comes to their rescue and becomes best friends with them.
In The Cook, the Thief, his Wife, and her Lover, Cory the pimp starts as a member of Spica's gang, but gets ejected after he resists Spica's brutal treatment of his girls. He ultimately joins the final confrontation with Spica.
In Get Shorty, Bear starts as a thug for Bo Catlett, but eventually switches to Chili's side.
In Oceans Eleven, Benedict is the main villain, but by Ocean's 13, he's convinced to support Ocean's gang, though he's still a ruthless bastard.
In Shaolin Soccer, local thugs beat on the monks several times before getting bested in a soccer game and joining their team.
In the documentary An Arctic Tale, the male polar bear is the main villain for most of the film, but near the end he shows compassion for Nanu, the polar bear protagonist, and shares his kill with her. Nanu later finds a mate, but we do not know if it is the same bear.
In Finding Neverland, though Emma is not an evil person, she is the only main character who is consistently antagonistic towards James Barrie. Her attitude changes completely as a result of seeing Peter Pan and Sylvia dying.
Derek and Danny in American History X after they become disillusioned with the Neo-Nazi movement.
The Cobra Kai students at the end of The Karate Kid. Made apparent at the start of the second film where Johnny accepts his loss and he and his friends leave the dojo. Their sensei, on the other hand, remains evil throughout the series.
In the 1982 film version of Annie, Miss Hannigan does a major one near the end. She had always been mean to Annie, a lush, and a less than ideal ward to the orphans. But when she realizes that Rooster chases after Annie hoping to kill her after she tears up the check, she tries to prevent him from doing so.
Number 2 attempts one in Austin Powers: International Man of Mystery before he's eliminated by Dr. Evil. Mini-Me successfully performs one in Austin Powers in Goldmember once Dr. Evil replaces him with Scott, and Dr. Evil himself makes the turn upon learning that he is Austin's long-lost brother Doug.
Also Fat Bastard.
The good guys in Escape to Athena easily convince Major Hecht to join them. To establish it before the turn, he is shown to be a Punch Clock Villain who is displeased to work with the SS.
In G.I. Joe: RetaliationStorm Shadow performs one after he finds out that Zartan was the one who framed him for the murder of his and Snake Eyes' master.
In Byzantium Darvell decides to turn on the Brethren and save Clara's life rather than blindly follow their rules any longer. In the end he asks to journey with her in the hopes she can forgive him for standing by all those years ago.
Elsa Schneider has one at the end of Indiana Jones and the Last Crusade. After watching Donovan shoot Indianaís father, she decides to give Donovan a fake grail on purpose to eliminate him. This allows Indiana to choose the true grail and use its holy water to heal his dadís injury. However, Elsaís redemption isnít enough to overcome the fact she had sided with the Nazis. In a Take My Hand moment, she risks trying to reach for the grail despite hanging over an abyss. She loses her life when her hand suddenly slips from Indianaís hold.
Magneto seems to have made the turn between X-Men: The Last Stand and his cameo appearance at the end of The Wolverine, because in this movie, Future!Magneto is firmly on Professor X's side throughout and ultimately makes a Heroic Sacrifice against the Sentinels during the climax.
Mystique eventually decides to give up her vendetta against Trask to prevent future bloodshed against Mutants. She also saves Nixon and his cabinet from Magneto.