SHIELD agent Stan Carter was a volunteer for an experimental drug, developed by S.H.I.E.L.D. R&D basing on Phencyclidine and aimed to increase strength and stamina. The project was interrupted because it was considered too dangerous (in fact, the drugs were driving lab rats crazy), and Carter, who considered this an affront and an authority abuse, violently resigned. There were no traces of drugs in his system, being totally purified, but nevertheless the drugs had already driven him crazy.
Carter then joined NYPD and become a detective. His partner died in a shoot-out while fighting some young criminals. This unchained in Carter an obsession to destroy people who abused of their authority and thus "sinned". He decided to "absorb the sins of the world". He wanted to purge evildoers from a society where nobody else could punish them (For instance, judges and lawyers giving criminals minor sentences, or priest protecting the criminals who confessed their sins). He bought a shotgun and equipment to become the Sin-Eater.
Carter was a good fighter, knowledgeable in martial arts and in guns, particularly shotguns. He was also a good detective and law enforcer.