Joseph Oberhansley, 38, stabbed Tammy Jo Blanton, 46, to death with a knife after breaking in to her home, cops say.
He reportedly used an electric saw to open her skull and gobble part of her brain in Jeffersonville, Indiana.
Oberhansley initially confessed to the crimes after his arrest in 2014.
But he now insists he is innocent, going into a 10-minute rant in a pre-trial hearing at Clark County Court.
Oberhansley called Tammy Jo’s allegations that he had raped her the weekend before “bull crap lies”.
He also suggested that investigators had missed the real murder weapon and that it couldn’t have been the “cheap little knife”.
Oberhansley was initially deemed mentally unfit to stand trial in October 2017.
However, last November a doctor found his competency had been restored at a psychiatric hospital.
Tammy Jo was found with parts of her skull removed under a vinyl camping tent draped over her bath in September 2014.
Cops also found a “plate with what appeared to be skull bone and blood” on it.
“Well I’m certainly taking notes of everything that he’s saying”
Prosecutor Jeremy Mull
At the time of the murder, Oberhansley was free on parole for fatally gunning down his girlfriend Sabrina Elder, 17, in a drug-fuelled jealous rage in 1998.
He also shot his mum in the back and fired at his sister before tucking the weapon under his chin and pulling the trigger to give himself a “partial lobotomy”, according to a psychiatrist.
He survived, but the bullet remained lodged in his frontal lobe.
Prosecutor Jeremy Mull said after Oberhansley’s courtroom rant: “Well I’m certainly taking notes of everything that he’s saying.
“And if there is evidence that is contrary to the things he’s presented now, presented at trial, I’ll certainly have these things to refer to.”
Oberhansley’s trial is due to start on August 19.