The Sin-Eater
Stan Carter was the acting chief of the NYPD Major Crimes Unit, who also had connections to S.H.I.E.L.D. as a former cadet in the police division. Back when the military organization was infiltrated by HYDRA, several scientists in league with the terrorist cell were developing a way to duplicate the Super Soldier Serum for their own ends to create super soldiers to further aggravate the wars they were hoping to engineer to keep humanity under their control. Stan naturally was unaware of the deception at hand, and was lead to believe that he was undergoing a test for refining and sustaining the strength enhancing effects of anti-psychotic medication such as PCT. Just as Stan was about to be injected with the serum and be placed under Faustus Method-mind control to serve HYDRA's whims, Director Nick Fury stepped in to pull the plug on the project due to there being negative side-effects earlier witnessed in the Gamma Mutates Hulk and Abomination. Stan was a little miffed that he couldn't get powers like he wanted, but he was also secretly relieved that he didn't end up going mad as was warned to him by Fury. Nonetheless, Stan decided to retire from S.H.I.E.L.D. and return to the NYPD in order to get away from the hectic super-governmental lifestyle and back to a more grounded in reality life. But before he left, he was briefly given an "immunization shot" by several of the same scientists to supposedly protect him from disease in the outside world, but little did Stan realize that the "flu shot" was in reality a small injection of the super-soldier serum, intended to turn him into a sleeper agent to be activated at HYDRA's whim when the time was right.