Stan Carter had been a SHIELD agent before entering the ranks of the New York Police Department. While he was with SHIELD, Stan had been a part of one of their experimental programs that had attempted to refine and sustain the strength enhancing effects of anti-psychotic medication such as PCT. The drugs had the desired effect, enhancing his physical attributes, though they took their toll on Stan and began to drive him insane. SHIELD soon pulled the plug on the program and Stan soon left SHIELD after the drugs had supposedly left his system.
He eventually made the rank of Sergeant in the NYPD. It was there that he met Captain Jean DeWolff. The two had an affair that no one else was aware of. Following their affair, Stan's partner was killed in the line of duty, which led to Stan's depression and drinking. Because of his drinking, the drugs' effects on Stan's mentality began to show up again. The effects combined with his deeply religious beliefs caused him to think it was his duty to destroy the sins of what he considered a corrupt society. These thoughts prompted him to become the Sin-Eater.
Armed with a shot gun, his first victim was none other than his former lover, Jean DeWolff. His reasoning for killing her was that she represented the very legal system he considered responsible for the death of his partner. He added to his belt her badge which signified that he sought justice.
Coincidentally, it was he who was put in charge of the DeWolff murder case. In the mean time, Spider-Man had learned of the death of his good friend (DeWolff) and went to Carter to offer his services in hopes of catching her killer. No one, not even Spider-Man could have guessed that Stan was in fact, the murderer.
As the Sin-Eater, he went on to murder Judge Horace Rosenthal, who Matt Murdock studied law under many years prior. Sin-Eater killed the judge because he felt he coddled the criminals. Murdock was also the sole witness to the murder of Judge Rosenthal, since he was in the same room when Sin-Eater struck. As Daredevil, Murdock was determined to bring Sin-Eater to Justice as well.
Fleeing from the courthouse where he murdered Judge Rosenthal, Sin-Eater was brought into his first confrontation with Spider-Man. Because of the drugs in his system, Sin-Eater was able to withstand some of Spider-man's toughest blows. During the fight, Spider-Man dodged one of the blast's from Sin-Eater's gun, but in effect, it struck an innocent bystander, thus bringing the Sin-Eater's death toll up to 3 victims. Seeing his Aunt May lying in the street, Spider-Man became too distracted and concerned for her, which allowed the Sin-Eater to get away. It was later revealed that Aunt May was fine; perhaps suggesting that she fell in the mass confusion during Spider-Man's battle with Sin-Eater. It should also be noted that Spidey saw DeWolff's badge on Sin-Eater's belt, which confirmed that he had indeed came face to face with DeWolff's killer. Spider-Man was now more determined than ever to stop his latest adversary.
Later as the Sin-Eater, Stan entered the confessional at a church to murder a priest who opposed capital punishment. Blinded by rage and determination, Spider-Man tore through the criminal underworld in search of any leads or clues that might lead him to the Sin-Eater. But he would not have to go looking for the Sin-Eater. The Sin-Eater would come to him.
The Sin-Eater later broke into the offices of the Daily Bugle, in search of his intended victim, J. Jonah Jameson. Unbeknownst to the Sin-Eater, Jameson and Ned Leeds were out of town. It was lucky that Peter Parker (Spider-Man) had been there to subdue the killer with a roller from a type writer, similar to the style Daredevil uses his billyclub. By knocking the killer out, Sin-Eater was unmasked and taken into custody.
In an interrogation room at police HQ, Emil Gregg, the accused killer, was being questioned why he had done it. He claimed to have been hearing voices that prompted him to do it; or, at least he thought he did. While Spider-Man was convinced he had caught the killer, Daredevil was not. Using his radar sense like a lie detector, he was able to notice that during the confession, Gregg's heartbeat proved that he was innocent; he was a compulsive confessor.
Upon Spider-Man and Daredevil searching Gregg's apartment for clues, they noticed a jimmied door knob at the apartment next to Gregg's. It was revealed to be Stan Carter's. Daredevil determined that the voices Gregg heard were Stan's confessions through the wall in the neighboring apartment, which convinced the mentally ill Gregg he was the Sin-Eater. Stan recorded the confessions on a tape recorder found in a closet with all of Stan's Sin-Eater gear. Spider-Man and Daredevil noticed that two shotguns were missing. One Gregg had taken, and one the real Sin-Eater must've taken. Realizing that Sin-Eater was still at large and that Jameson was his next intend victim, Spider-Man and Daredevil raced to Jameson's house where Betty and Marla Jameson were staying. Sin-Eater had planned to kill Jameson because he opposed masked vigilantes, but was going to kill Betty as a message to Jameson when he returned from assignment with Ned Leeds. He also planned to return and kill Jameson when he got back. Arriving in the nick of time, Spider-Man beat Stan to a pulp for playing him like a fool. If not for Daredevil's intervention, Spider-Man may have done the unthinkable.
Later at the NYPD, a mob of angry citizens arrived to kill Stan. Daredevil tried to get Spider-Man to help him stop the mob, which an angry Spider-Man promptly refused to do, since he felt Stan deserved whatever happened to him. Outmatched by the crowd, Daredevil, in an act of desperation, called out "Peter!" and Spider-Man arrived just in time to save Daredevil and Stan after he came to his senses.
It was later revealed that Emil Gregg, the false Sin-Eater, had gave anonymous false interviews to Eddie Brock when he believed that he himself was the Sin-Eater. This was Brock's big break, the high point in his editorial career. When Spider-Man captured the real Sin-Eater, Brock was exposed as a fraud and fired from the Daily Globe, his marriage ended, and his father disowned him. This drove Brock to want to commit suicide. He was at a church praying when the symbiote found Eddie and formed the composite entity, Venom. The symbiote had picked up on Eddie's hatred for Spider-Man and his anger for Spidey catching the real Sin-Eater; and thus the two shared a mutual hatred. It's suggested that if not for the Sin-Eater, there never would have been Venom.
Sometime later, Stan was released from prison when doctors and SHIELD decided he was sane and the drugs were out of his system. He was now walking with a limp, had a stutter, and also had partial deafness as a result of the beating Spider-Man had given him when he attempted to kill Betty. He had also developed the Sin-Eater persona as a split personality that was unknown to the doctors and SHIELD; only he could see and communicate with this dark persona. He was now also jobless as well, which prompted him to write a book so he could make a living off the book's sales.
Spider-Man was enraged that Stan was released and attempting to make money off a book, but was shocked to find Stan in the condition he had left him in after their previous battle. Spider-Man then decided to leave Stan alone and was hesitant in a battle with Electro that he might end up doing to him what he did to Stan. Meanwhile, Stan seemed to lose to his Sin-Eater personality, and had to be taken down by the shots from at least six policemen at a television studio where Spider-Man was fighting Electro. Spider-Man was able to conquer his fear that he might do to Electro what he did to Stan and dropped Electro off to one of the policemen that were at the scene of the final Sin-Eater showdown. Spider-Man came to realize that the Sin-Eater persona became a seperate personality from Stan's own conciosness. It seemed that Stan Carter had conquered his Sin-Eater personality after all, since the gun he had been using as Sin-Eater was unloaded, which was later revealed to have been Stan's doing. As he lay dying, the Sin-Eater persona had finally disappeared, and Stan finally believed he could begin living his life.