John DeMarco was a crooked member of the Gotham City Police Department's SWAT Division, as well as a murderer.
John DeMarco was one of the corrupt cops in the GCPD, who acted as a member of the SWAT Division. He was convicted with and proven guilty of drug use, and was apparently placed in jail for a few years. By Christmas Eve, due to Internal Affairs planning to investigate his unit, as well as one of his teammates, Nate Ramo, who planned to undergo a plea bargain to expose his unit's corruption (as IAD lacked sufficient evidence to expose his unit otherwise), DeMarco decided to silence him by ramming him with his car. However, the blizzard during that time forced him to lose control of the vehicle and crash near the GCPD Building after he successfully conducted his hit and run.
DeMarco eventually managed to flee before the GCPD managed to investigate the crime, as well as searched Ramo's House for anything incriminating about his unit. Batman learned of the hit and run from a Police Dispatch by Officer Meyer, investigated the murder, and deduced DeMarco's role due to residue paint from DeMarco's earlier crash (as his car had carried a distinctive paint job). DeMarco then reconvened with his SWAT unit, although they were aghast that he did the hit and run in front of the GCPD, as that made their being found guilty of corruption more than certain, but DeMarco was confident that they'd be back at work after the investigation was over.
Unfortunately for DeMarco and his unit, Batman tracked him down to their meeting area and proceeded to subdue him. Batman then interrogated DeMarco, and proceeded to inform him that since he committed murder on a fellow officer, that meant that his prison sentence had increased to life imprisonment, although DeMarco implied that he'd try to have one of his friends remove the evidence and ensure that he went scot-free. Batman taped DeMarco's confession, vowed that he would ensure that the evidence wouldn't disappear, left him and his unit to be apprehended by the GCPD, and also had Alfred send the taped confession to them as well. It could be assumed that this evidence started a chain reaction that eventually revealed the massive corruption present within Howard Branden's SWAT Division and allowed Internal Affairs to finally convict them of at least some of their crimes.