In the non-canon Wolverine: The End, Paul Jenkins has Sabretooth, one of Marvel's most deranged and sadistic murderers, renounce his former life and become a born-again Christian.
Shows up in most of, if not all Chick Tracts.
FIRE AND FAITH MORROWIND SERIES
The French-Belgian comic book series Odilon Verjus is about a former pimp who has become a Catholic missionary, though in his case, while the embrace of the faith was sincere, there was no particular desire for atonement.
He finds that the Bible, Torah, and Quran are all equally valid religious texts in the eyes of God, and after his conversion, becomes a stronger, more motivated, and determined character for the rest of the series, as he now believes he has something worth fighting for.
Cerebus the Aardvark: The Trope Namer for all things that start happy and fun and end in the most horrific way possible, the series actually subverts its own trope when the titular protagonist, Cerebus, near the final arcs of the series, undergoes a religious experience similar to that of the comic's author, David Sim.
By the end of the movie, he has started believing in a higher power and has gotten a lot more friendly as a result.
In Highlander: The Search for Vengeance, Colin McLeod spends most of the film as a thoroughly unlikeable Hollywood Atheist.
What makes this example interesting is the contrast provided by the other main character, Akanemaru, a carver of Buddha statues, who starts out as a decent enough guy, but gets involved with the politicians who are trying to use Buddhism as a state religion to control the masses & turns into a vain Jerkass who thinks nothing of sacrificing the lives of countless workers to build his giant bronze penis extension Buddha.
In another Tezuka work, Phoenix: Karma, widely considered to be his greatest masterpiece, Gao, a murderous bandit, is inspired to become a pacifistic, nature-loving artist by the teachings of the Buddha.
Garai from Osamu Tezuka's MW started out as a violent gangster who molested a nine-year-old boy, but he became a priest to try to atone for his evil ways.