Bill Patterson

Bill Patterson is the third villain of the X-Files episode, "Grotesque" - sharing said role with John Mostow and the Gargoyle : like Mostow he continues a trend in the series of serial-killers with supernatural abilities as well as a tendency for figures of authority to reveal dark secrets or alternative motives.

History
Bill Patterson was an FBI agent who had been investigating Mostow and his grisly murders for three years, eventually capturing him but growing obsessed with the case as a result.

Despite Mostow's imprisonment a string of murders almost identical to those Mostow was infamous for began to manifest and Patterson decided to get Agent Mulder on the case, in the process Mulder started to show the same behaviors as Patterson did, only at a much quicker pace, growing obsessed with the case and in particular the Gargoyle - a demon Mostow claimed was possessing him and responsible for all the murders.

Eventually Mulder goes to Mostow's old hiding spot and finds Patterson's old partner, Agent Nemhauser, murdered and mutilated - Patterson arrives on the scene and Mulder confronts him, realizing he was the copycat killer : Patterson attacked Mulder and tried to flee the scene only to be shot by Mulder on the rooftop.

Patterson is arrested and locked up for his crimes, with Mulder believing that Patterson had "stared too long into the abyss" while investigating Mostow, however as he is put in his cell Patterson claims innocence and we see that he has drawn an image of a Gargoyle in his cell, just as Mostow did.