PRISON guards feared Harold Shipman would continue his killing spree behind bars when he told one inmate: "Remember I am a doctor; I know where to cut you," it has emerged.The former GP made the threat after he was imprisoned in 2000 for the murders of a string of his patients.Prison officials warned the serial killer could kill again after he began 'grooming' inmates in the hospital wing of the high security jail where he was kept for his own protection.