It was the computer geek David Willerman. It was obviously him that attacked Peter back in 72 and discovered his mother dying. All the murders were recreations of his mother's suicide/homicide, while "his mommy complex" stems from his part in her demise. His final victim must be Erica Sanderson, Peter's daughter...so that Peter could feel the pain and suffering David does for not helping his mother. During which his mania leads him to murder Det. Andy Wagner & Capt. Frank Sedman with a series of sharp razor slashes to their throats. Shrink Kathy Sheppard (who discovered David was an out patient of the local hospital) lets him know she feels sorry for him as he's loss has driven him mad with grief But despite her pleas that there's no need for anymore bloodshed (since it'll never bring his mother back) and he doesn't have to carry on with his carnage, David makes it clear he can't stop himself so is shot dead by Peter.