David Cheres, 21, kicked and slashed at 43-year-old Tiprat Argatu with a knife and scissors at her home in Whitechapel, east London, in the early hours of January 24 last year.
He then hid naked in a nearby bin store before he was arrested by police where he told an officer: “I’m sorry for what I did.”
Cheres, of Ellen Street, Whitechapel, who is mentally ill, had admitted manslaughter by diminished responsibility but denied murder.
A jury deliberated for five and a half hours to clear him of the more serious offence following a trial at the Old Bailey.
Previously prosecutor James Dawes KC has described how Cheres had battered and kicked the victim to death in her bedroom over a period of about 50 minutes.
He told jurors: “Tiprat suffered a savage and prolonged attack at the hands of this defendant.”
He stamped on her and slashed and stabbed her head and neck with a kitchen knife and a pair of long scissors.
The victim was unarmed and offered little resistance to the onslaught in which the scissors were bent in half.
Mr Dawes said: “Tiprat appears to have held up her hands in order to try to defend herself because some of the stab and slash wounds were on her hands and fingers.
“He stamped on her neck, breaking bones and her larynx. He smashed the bones in her face and neck.”
“You may conclude that this attack took time, took effort and it was relentless and merciless.”
A pathologist concluded the cause of death was severe blunt impact trauma to the head and neck.
Police were called to the scene and discovered Cheres walking naked outside shivering with cold and with cuts and scratches on his body.
Asked where he had come from, he indicated a bin cupboard with a duvet and pillow inside.
Mr Dawes suggested the killing was connected with defendant’s feelings about “the way his life was going” and the loss of his friendship with the victim’s husband.
Cheres had been friends with Mrs Argatu’s spouse, who had discovered he was having sex with her just hours earlier, the court heard.
Mr Dawes said: “Perhaps he blamed himself or her for the breaking of this friendship.
“What is clear, he directed this outburst of anger at Tiprat – not at himself – in the small hours of the morning, about 12 hours after (her husband) had discovered that they were having an affair.”
The court heard that the victim was originally from Thailand and her husband was Romanian, and the attack took place at a flat in Ellen Street, Whitechapel.
Cheres, also of Ellen Street, Whitechapel, was remanded into custody to be sentenced on February 1 next year.