Elliot Turner, 20, is on trial at Winchester Crown Court for the murder of aspiring model Emily Longley, 17, who he allegedly strangled in a jealous rage at his parents' Dorset home in May last year.
Along with his parents Leigh and Anita, he is also charged with perverting the course of justice.
All three deny the charges.
When questioned in court about a letter Emily wrote during their Isle of Man trip in March last year in which she asked him not to say he would kill her, he said "I admit that I said I would kill her. I didn't literally mean that,'' the Bournemouth Echo reported.
He admitted to being verbally aggressive towards her, to hitting her after a public argument days before she died, to telling friends he had killed her with a mallet and to punching the wardrobe at the hotel following an argument while on the Isle of Man trip.
Turner denied holding Emily against a wall by the throat and becoming obsessive while she was on girls nights out with friends, said the newspaper.
"He also told the court how he "instantly clicked'' with Emily during a double date the night they met and how they shared similar interests.
The court has previously been told the 20-year-old had become increasingly suspicious Emily had been having affairs during their four-month relationship.
Turner's friend, Luke Ashford, admitted to the court that he had unprotected sex with Miss Longley six days before her death and he believed the pair had managed to keep their sexual relationship from her boyfriend.
Turner was arrested following Emily's death on May 7 last year but was then released on bail.
He has claimed Emily become angry and lashed out after he asked her about self-harming issues the night before she died.
He said he had pushed her on the neck to get her off him and she calmed down before they went to bed.
When he awoke later that morning he found her dead.
The court has been told Emily's body had injuries consistent with asphyxiation, Turner had scratches on his arm and Emily had his DNA under her fingernails.
Emily was born in Britain but her family moved to New Zealand when she was nine and she was raised in Auckland, where she attended Hauraki School, Westlake Girls High and Takapuna Grammar.
She had returned to study in Britain and lived with her grandparents in Bournemouth while continuing her studies there.
