Andrew Hasler, 28, was killed in the onslaught at his home in St Ives, Cambridgeshire, on 8 January.
Matthew Sharpe, 40, also pleaded guilty to the attempted murder of his 27-year-old ex-girlfriend Charna Knights, who was saved when a neighbour intervened.
Cambridge Crown Court heard he found Sharpe on top of her, with Mr Hasler lying wounded nearby.
Mr Hesler was repeatedly stabbed and died from a single wound to the heart.
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The neighbour, who had heard "a commotion", managed to pull Sharpe off Ms Knights and kick the knife out of his hand before restraining him.
Sharpe managed to get away but was arrested after jumping into the river in St Ives. Police said he was found to have self-inflicted injuries that required hospital treatment.
Det Insp Lucy Thomson paid tribute to the neighbour, who she said "put his own life at risk in order to help Charna".
"His actions that day ultimately saved her life."
In a statement, Mr Hasler's family described him as a "much-loved son, brother, grandson, nephew and friend".
"Kind-hearted and living his life to the full, he came into the world early and has left us far too soon."
Appearing via video-link from Bedford Prison, Sharpe was told by Judge David Farrell: "There's only one sentence I can impose and that's life imprisonment.
"I will hear submissions to determine the minimum period you must serve."
Sharpe, of Eynesbury, St Neots, is due to be sentenced on 7 April.
A man who launched a brutal stabbing attack in which his ex-girlfriend was seriously injured has admitted murder.