Alex Burnett, 32, from Westlea View, Clowne, is accused of murdering 52-year-old mum-of-three Lee Hendry on 13th September 2011.
The pair were reacquainted the day before after more than a decade when Ms Hendry changed her profile picture on Facebook and Burnett asked to be her friend.
A pathologist, who completed a post mortem on Ms Hendry, told the court she was found with bruising around her neck consistent with strangulation and sex play.
Ms Hendry had a keen interest in vampires and her sister, Maxine Bonser, told the court she liked being dominated during sex.
Burnett, who denies the murder of Ms Hendry, from Brimington, Chesterfield, was called to give evidence on the fourth day of his trial at Nottingham Crown Court.
He told jurors he would often strangle her during sex and that she introduced the knife after selecting it from the kitchen.
“She wanted the knife held to her throat at the same time I would normally be doing the strangulation,” he said.
“It would be pointed towards her throat. During sex she was showing me how to hold the knife to her.”
“She was holding the knife with my hand over the top.”
He added: “I went to roll her over on to her back and at that point the knife went into her neck.”
“It happened instantaneously.”
Burnett said he was ‘extremely drunk’, ‘panicked’ and fell asleep next to her before moving her body on to the floor.
Peter Joyce QC, prosecuting, said Burnett ‘watched her die’ then did nothing to help her as he went to sleep by her side.
Burnett handed himself in to Chesterfield Police Station around 8pm on Tuesday 13th September.
Mr Joyce asked him why he told officers he had committed a murder and not that a terrible accident had happened at his home.
“Because I did not think anyone would believe me,” replied Burnett, who denies the charge of murder.
The trial continues.