Can anyone give me historical (or at least purported) examples of people who after being notoriously evil mended their ways? I gather a few saints started out as pretty sinful, but I’d draw a distinction between the merely dissolute and the truly monstrous? Anything on the level of a Hitler or a Stalin becoming a monk and spending the rest of their life in penitence?
In current times there is the Liberian Cannibal Warlord, General Butt Naked: General Butt Naked - Wikipedia
He found the lord, which doesn’t say much for his rational capacity, but at least he has stopped eating people. He wasn’t Stalin or Pol Pot, but he probably lead to quite a few deaths.
Mitsuo Fuchida led the first wave of attacks on Pearl Harbor (OK, maybe not evil so much as being an enthusiastic officer), and after the war became a Christian missionary.
Lee Atwater was one of the most vicious and ruthless campaign leaders ever, most notably in GHWB’s 1988 campaign. Following his brain cancer diagnosis, he devoted his remaining time to making apologies.
Charles “Chuck” Colson was a notorious henchman for Richard Nixon and played a big part in the Watergate scandal. Later on, he turned his life around and started Prison Fellowship, a Christian charity.
A Godwin right out of the bat. It is hard to find people at the level of Hitler that reformed.
But if we lower the bar then I propose and did remember about Emperor Ashoka of India (304–232 BCE). He became a devout Buddhist and gave up on making warfare after seeing the destruction and many deaths that took place when he conquered Kalinga.
There were several top Nazis that live perfectly normal lives after the war. Whether that they had reformed in their hearts is, of course, open to question.
John Newton once captained a slave ship, and later became an ardent abolitionist. He’s best known for writing “Amazing Grace.”
Nicki Cruz, was a leader of a New York street gang and a notoriously violent person when he became a Christian due to the ministry of street preacher David Wilkerson, as portrayed in the movie “The Cross and the Switchblade”.
Takashi Nagase was an interpreter in the Japanese Army, who participated in torture and brutality on the Thai-Burma railway. After the war he became a devout Buddhist priest and tried to atone for the Japanese army’s treatment of prisoners of war. he made more than 100 missions of atonement to the River Kwai in Thailand.
The story of his treatment of, and subsequent reconciliation and friendship with, one of his victims Eric Lomax is the basis of the 2013 film The Railway Man.
Augustine was once a pear thief and a womanizer, but eventually found God, with the help of his mother, Monica, and because a saint. Got his mother sainted too, IIRC. “Lord, make me chaste, but not just yet.”
So, as far as reforming in ways other than “but then he found religion”, how about Frank Abagnale - one of the most famous con artists and swindlers of his day, who went on to help big companies protect themselves against the very kind of scams he used to pull on them?
Or David Brock, who spent the '90s as a right-wing hatchetman and character assassin, and then founded Media Matters to expose people doing the stuff that he had done?
Or Alan Chambers, president of “pray the gay away” group Exodus International, who shut the group down and publicly apologized for the harm they’d done to gays and their families?
Probably Hans Frank: http://en.wikipedia.org/wiki/Hans_Frank
Jack Abramoff also is up there: http://en.wikipedia.org/wiki/Jack_Abramoff
I always get him mixed up with Jeb Magruder (who also went into the clergy after Watergate), but one of them was interviewed after the death of Mark Felt (aka ‘Deep Throat’), and made the ridiculous assertion that Felt shouldn’t have talked to Woodward and Bernstein, but instead should have reported Nixon’s abuses up the normal chain of command.
If that was Colson, it sounds like the Kool-Aid hasn’t completely worn off.
The prototypical “found Jesus” turnaround was St. Paul, who fervently persecuted and jailed the followers of Jesus in and around Jerusalem in the early years of the movement. He had his “Damascus Road” experience on the road to Damascus where he saw the light when he saw a great flash of light. (Told you he was the prototype.)
I wouldn’t really call him evil in the beginning, just - overenthusiastic about religious purity. He got better.
“Governor-General of the General Government” sounds like the start of a Monty Python sketch or something out of Joseph Heller.
The sincerity of Frank’s religious conversion, and his true penitence, is not obvious to this observer. He would not be the first condemned prisoner to make the claim in hope of avoiding execution.
Yeah, like I always say about Hitler: “At least he wasn’t a pear thief.” 
A case could be made for Asa Earl Carter. The guy started out as a Klan member and die hard segregationist. He wrote George Wallace’s infamous 1963 inaugural speech. After public opinion shifted against him on the issue a few years later he disappeared and came back as Forest Carter.
George Wallace would be a candidate.
Wallace himself apologized later in life, after his paralysis IIRC, for his hateful race-baiting earlier in life. Many other segregationists softened their hearts as well, but he was probably the most notable both as a leader and as a repenter.
There must have been cases in South Africa of former apartheid supporters publicly reversing themselves too.