Even before The Beast forced its way into mankind's collective consciousness and forced almost all of humanity - save for John Constantine and a few other powerful mages - to commit appalling atrocities, Peter Gill was a bad, bad man. A serial killer, even, who preferred to murder with a knife rather than a gun because 'killing with a gun is like shagging with a rubber on'. Peter Gill seems to possess powerful psychic abilities that he used to gain information on almost everyone. He used these information to verbally play tricks on his victims, sometimes even mentally torturing them with it before death.
Most ordinary people, upon waking from The Beast's control, forgot the terrible things they'd done as a coping mechanism, but Peter remembered it all. And when he met an amnesiac Constantine, he became fascinated with him, realizing that his was one of the few minds that The Beast never touched. Gill tracked Constantine down and attempted to make him into a serial killer too, forcing him to murder Chas Chandler and his family, but Constantine was able to distract him long enough to grab a shotgun and shoot Peter in the chest. It was implied that Peter had been directed towards John by Rosacarnis, who wanted to frighten John into doing her bidding.
His soul is later feasted upon by Rosacarnis in Hell.