A judge has called for an inquiry after a convicted murderer who killed his cellmate at Manchester Prison was jailed for life.
Psychologists thought McNally "took pleasure" from the killing
Clement Peter McNally, 28, admitted manslaughter on grounds of diminished responsibility when he appeared at Manchester Crown Court on Monday.
He had killed Anthony Hesketh, of Little Hulton, Salford, who was in jail for driving and alleged drugs offences.
The Prison Ombudsman has since begun an inquiry into the incident.
McNally, formerly of Ashton-under-Lyne, was already serving a life sentence at the prison, formerly known as Strangeways, for stabbing a friend to death at a party.
The court heard McNally had strangled Mr Hesketh with a piece of a T shirt.
Psychologists had since concluded that he suffered from "a severe personality disorder and obsessive compulsive thoughts and urges to kill," said Alan Conrad QC, prosecuting.
He added: "It was made clear to the psychiatrist by the defendant that the killing had brought him pleasure and that he had killed Mr Hesketh as an act on behalf of Satan."
He said psychologists concluded he was likely to harm others and felt no remorse.
Mr Hesketh was serving a short sentence for driving while disqualified but was also on remand for drugs offences.
Mr Justice Davis told McNally the circumstances of the prison's decision should be investigated.
He added: "The decision to allow you to share your cell with Mr Hesketh has been disastrous in its consequences.
"It may be that at the time the prison authorities did not know then what we all know now.
"But I have to say I regard this particular act of homicide as extremely callous and extremely ruthless."
A Home Office spokeswoman said the Prisons and Probation Ombudsman would investigate the incident.