Roy Allison Snr, 65, confessed that he wanted to kill his son's friend in a nine-page letter sent to police and a local newspaper, before taking his own life.
His son, Roy Allison Jnr, a professional fighter, died in mysterious circumstances the day after his 28th birthday party on March 21.
In the early hours of Sunday, Mr Allison Snr, 65, stabbed Duncan Bell, 36, in the heart at the home the father and son had shared in Hetley, Orton Goldhay, Cambs, believing he was responsible for supplying Mr Allison Jnr with drugs that led to his death.
Later the same morning police found Mr Allison Snr's body hanging at Peterborough Crematorium, where he had said goodbye to his son.
In his letter, Mr Allison Snr claims that Mr Bell drunkenly admitted supplying his son with "Ecstasy-like" drugs despite knowing that he had high blood pressure and admitted guilt for the death.
Mr Allison Snr wrote: "When I lost my son, my grief was such that I just wanted to die and join him. I was in shock and disbelief. A few weeks on, I realised no matter what I wanted, I had to do for my son the right things and put our house in order. Duncan Bell, known recently as my son's best friend, is one of the biggest dealers of drugs in this area. Duncan caused his death."
Mr Allison Snr claims he bought the same drug given to his son to help him to piece together what led to his death.
He added: "When the funeral was over, Duncan became his true self and talked too much?Duncan took our whole existence?and for that he will lose his life at my hand. Like he did my son, I will do to him and take it all away from him. I take my own justice.
"I believe it is right for me to rid this scum from the community...He will not have all the things he denied my son. It won't bring my son back but the man who caused it will not walk the Earth any more."
An inquest into the death of Mr Allison Jnr, a mixed martial arts fighter who was said to be in peak condition, was due this month but has now been delayed.
Mr Allison Snr also claims in the letter that Mr Bell was jealous of his son's long friendship with another man, Grant Maker. He claimed that on the night of his son's death Mr Bell forced Mr Maker to leave the house so he could not be saved.
The letter was postmarked Monday, but arrived at the offices of the local newspaper on Tuesday. The editor of the newspaper contacted police immediately but both bodies had already been found.
It is understood that Mr Allison Snr had not raised his allegations about Mr Bell with Cambridgeshire Constabulary.
Det Insp Liz Mead said: "Inquiries are continuing into the tragic set of circumstances that appear to have led to these two deaths."
Mr Bell's family said in a statement: "Some people make such a difference in our lives, Duncan was one of those people. By simply being who he was he made the world a better place."
A close relative claimed Mr Allison Sr was an alcoholic and an "evil man". He said: "He was determined to find out who gave (his son) the drugs and kill them."
