Atonement Detective

A crime-solver in a Police Procedural who has killed and is The Atoner because of this. Their method of atoning is to catch other criminals. Because they can think like criminals, they are often better at this than the average cop.

Interestingly, this motive even applies to people whose killing was perfectly legal—people who served in the military, the FBI, or a spy agency. It also applies to detectives whose killing, and thus whose need to atone for it, is ongoing.

A Vampire Detective Series is a supernatural version of this.

Compare Recruiting the Criminal.

Beware if one of these has a Face Heel Turn.

Examples of Atonement Detective include:

Anime and Manga

Richard from Monster.

Literature

In the "Father Brown" stories by G. K. Chesterton, Father Brown (the series detective) meets and bests a thief named Flambeau, who, because he wants to atone for his crimes, then assists Father Brown in solving crimes by providing information about how professional criminals work.
- Sort of. The Blue Cross, the first Flambeau story has him as the villain, trying to steal a valuable church relic from a 'country bumpkin' priest... who happens to be Father Brown. Father Brown has heard so many confessions that he knows horrific criminal tricks that Flambeau has not even heard of.
Skulduggery Pleasant. Since he is actually Lord Vile this is not surprising.
Larry Niven 's Gil Hamilton. He lost an arm in an accident, had it replaced with a transplant, and then later found out his new arm came from a man murdered for his body parts. He joined the Amalgamated Regional Militia (aka ARM), the agency which polices illegal body harvesting.
Commander Vimes has killed a lot of people, and even though they were mostly self-defence, he's aware that he wasn't really thinking about that when he killed them. He explicitly tells The Dragon in Snuff that he recognised him as a killer the moment he saw him - because he's used to seeing a killer's face every day in his own mirror.

Live-Action TV

Leroy Jethro Gibbs in NCIS (Former Marine sniper) Gibbs also killed his wife's and daughter's murderer
Mac Taylor in CSI: NY (Former Marine)
Seeley Booth in Bones (Ex-sniper) He actually voiced this as his reasoning for joining the FBI.
- A supporting character on Bones was secretly working for a villain while he was doing the lab work to solve other crimes...
Horatio Caine in CSI: Miami (Kinda) He tries to catch as many murderers as subject him to Suicide by Cop. Preferably not by Suicide by Cop.
Ezekiel Stone of Brimstone killed the man who raped his wife. When Ezekiel was later killed in the line of duty he was sent to Hell because, no matter how deserving of death the rapist had been, Stone's actions had been motivated by anger and the need for revenge instead of a desire for justice. The Devil sent him back to Earth to hunt down and return 113 escaped souls with the promise of a chance at entering Heaven (Not a guarantee, just the offering of a chance) if he captured all the escapees.
He never really killed, but Sebastian Stark of Shark was once a defense attorney . However, after one of his clients who he managed to get acquitted went on to kill his wife shortly after, Stark switched teams and becomes a prosecutor.
Likewise, Patrick Jane of The Mentalist didn't kill anyone, but his family was killed by a serial killer whom he'd offended with his fake psychic routine. Assisting the police is his way of making amends for this error, as well as his former career of conning people.
The Cowboy Cop protagonist of the show Maou became a detective out of guilt over having accidentally killed someone when he was younger (and having been let off scot-free because his rich and influential father pulled some strings).
Castle: Captain Montgomery spent his career trying to atone for his actions as a rookie, when among other things he was complicit in the death of Beckett's mother.
In Body of Proof, former neurosurgeon Megan Hunt is trying to atone for killing a patient on the table after neurological damage causes her hands to numb occasionally. She becomes a medical examiner because she can't kill anyone if they're already dead.

Video Games

The Suikoden series has the Oboro Detective Agency in Suikoden V.
A less severe example in Ace Attorney is Ron DeLite, a former thief, opening a security business that helped other businesses defend themselves against thieves. Of course the twist is that he then turns around and sells the plans to other thieves. At least his wife's in on it this time.
Cole Phelps was a Marine in the war. Specifically, he was The Neidermeyer who ended up leading an attack on Japanese civilians.

Real Life

Career con man Frank Abagnale, Jr. (on whose life the Leonardo DiCaprio film Catch Me If You Can was based) eventually settled down and became a high-profile securities consultant who specialized in the sort of crime he used to commit. It is also notable that to this day he has never taken money from law enforcement for his consultation, even refusing reimbursement for travel expenses.