Paul Thompson, 37, launched punches, kicks and stamps after he witnessed his victim become aggressive towards a woman.
Newcastle Crown Court heard that Thompson acted in defence of the victim's then partner but left the man with a broken nose and a laceration to his liver after the attack in June last year.
Prosecutor Michael Bunch told the court that the victim has since died from an unrelated cause after the incident in Washington.
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Mr Bunch said: "Police became involved at 4am after neighbours called the ambulance service having heard noise of a disturbance.
"A man was found bleeding from his nose and mouth. He was conscious was transported to the RVI.
"When asked by police, he declined to provide an account of events."
However, subsequent examination of CCTV showed the victim and his partner enter an address on Waterloo Walk with Thompson.
Mr Bunch added: "The complainant and his former partner emerged from the flat.
There is a dispute ongoing between the two at which point he was struck by the defendant.
"The defendant ended up on top of him. The defendant struck him repeatedly with both fists before stamping on him.
"He stamped on him with full force before punching him and kicking him further.
"The defendant was located by police a couple of hours later. On arrest he told police he was sticking up for the female."
Thompson, of Perth Road, Plains Farm, Sunderland, later admitted grievous bodily harm with intent.
In defence, the court heard that he had shown genuine remorse for his actions.
His counsel, Jamie Adams, also read out a letter on behalf of his partner which stated his behaviour was not reflective of his character.
The letter included detail of a recent incident in an ASDA store that saw him come to the aid of an elderly woman as well as being a caring step-dad for herdaughter.
Judge Christopher Prince acknowledged his remorse but sentenced him to three years behind bars.