"A knife cuts away the parts of something not needed. This is the year of the knife for me!”

Those are the twisted rantings from the disturbed mind of O.J. Simpson’s son Jason, who kept a secret diary in the months before Nicole Brown Simpson and her friend, Ronald Goldman, were slashed to death.

On the 21st anniversary of the murders, The National ENQUIRER has exclusively obtained that journal – which provides a startling glimpse into the life of demon-ravaged Jason.

The son of the former NFL star has been sensationally fingered as an accomplice to the 1994 murders, as this publication previously reported.

In a bombshell turn of events, Los Angeles private investigator Michael Alexander Martin identified then-24-year-old Jason as being at the grisly Brentwood, Calif., crime scene.

Michael said he was parked about 40 yards from Nicole’s tony townhouse on an unrelated stakeout that fateful night.

Around 10:15 p.m., he said he saw O.J. – and his son – park O.J.’s infamous white Ford Bronco in an alley behind Nicole’s home.

O.J. exited the car and slipped on gloves, claimed Michael.

Ten minutes later, O.J. returned with a bundle of clothes under his left arm.

Father and son then drove off into the night, Michael alleged to The ENQUIRER in a world exclusive interview.

Texas-based private investigator William Dear – who has solved cold cases for the FBI – obtained Jason’s diary from an abandoned self-storage unit in Los Angeles.

In one chilling entry, Jason wrote: “My laugh … was the laugh of a madman. The blood I mentioned previously, coming from my arm? A direct result of madness.”

In another, he penned: “I can’t help but think of the story of Dr. Jekyll and Mr. Hyde. A man that when induced with a chemical becomes a madman.”

The journal is also laced with descriptions of his “self-destructive behavior” and drug use, including cocaine and ecstasy.

“I have fun when I’m slipping … drinking,” read one disturbing entry.

He added: “I saw myself on my trip and I wasn’t disappointed with what I saw. I was, however, uncertain about certain aspects of myself.

“My insecurity and knack for letting it become uncontrollable and ultimately contributing to the demise.”

William commissioned Texas handwriting expert Don Lehew to analyze Jason’s journals, who concluded that “no doubt” they match his handwriting.

The hidden journals are even more disturbing when considered alongside other contents that William found inside the storage unit.

He told The ENQUIRER the cache of evidence included a knife that Dr. Vincent DiMaio, a forensic pathologist, found to be consistent with Nicole and Ron’s wounds.

William also secured a photo of Jason lying in bed with his dog while wearing a black cap – just like one found at the crime scene that was determined to have traces of dog hair!

What’s more, Jason refused to take the powerful psychotic drug known as Depakote in 1994, his ex-girlfriend told William.

A prescription had been written for Jason after he had been psychiatrically treated for violent impulses and behavior, William has alleged.

He said a report from Cedars-Sinai Medical Center dated January 1994 – five months before the murders of Ron and Nicole – identified Jason as a patient who “feels like he’s going to rage.”

One journal entry elaborated: “Here I sit. On a hospital issued, pink, plastic and wood chair. On level two. Yes level two. I survived 21 years and two suicide attempts, countless drug-filled nights.”

“I will now not stop until final justice is given to the victims,” private eye Michael told The ENQUIRER.

“If Jason Simpson was ever charged in this case, O.J. could be arrested and charged with aiding and abetting. There is no double jeopardy if it were to be a charge other than murder.

“I need to continue this and make right for all what I should have done 20 years ago.”

For his part, O.J. – now jailed in Nevada on an unrelated burglary case – has declared of his son: “The theory is just one of a thousand regarding the murders, and the LAPD has fully investigated this and felt it has no merit.”