Vincent Tabak claims it was "pure chance" that he strangled Joanna Yeates, a court heard today.
The killer misread her signals by trying to kiss her after she invited him in for a drink, his counsel said.
Tabak says he put his hand to his 25-year-old victim's throat after she protested at his advances.
His full version of events was heard for the first time as defence QC William Clegg said Miss Yeates's death was tragic misfortune.
Tabak "completely misread the situation" before "Joanna went limp", Mr Clegg claimed.
"It was pure chance that Vincent Tabak and Joanna Yeates ever met on December 17 last year," the barrister told Bristol Crown Court.
Miss Yeates's boyfriend Greg Reardon fought back tears in the public gallery as Mr Clegg said Miss Yeates was "bored and lonely" on the night of her death.
She invited Tabak into her flat after smiling at him as he walked past her kitchen window, Mr Clegg said.
"If Joanna Yeates had stayed for one more drink in the Ram pub, she'd be alive today," the barrister said.
"If Vincent Tabak had left half an hour earlier to go to Asda, as was his intention, he wouldn't be standing in the dock now.
"Tragically, Joanna didn't stay for a last drink and went home, arriving around 8.30pm.
"Joanna went into her flat. She took off her coat, she took off her green fleece that she was wearing under her coat and put it on a chair, she took off her boots and she went into the kitchen.
"When in the kitchen she switched on the oven, presumably to pre-heat it before doing the baking she had been researching on the internet earlier and had spoken about to friends.
"She opened one of the two bottles of cider she had bought at Bargain Booze and maybe had a drink from it.
"She was bored and lonely."
Miss Yeates's parents, David and Teresa, held hands in court while Tabak kept his head so low in the dock that he was barely visible.
Mr Clegg said Miss Yeates and Tabak were virtual strangers - but were both at loose ends at their flats in Clifton.
He added: "They had never really met before, other than a nod as they would pass in a passage outside their house, a sort of acknowledgement one might give to a neighbour that you recognised but had never really spoken to.
"Indeed, her cat had had more contact with her neighbours than she had.
"Vincent Tabak was also home alone and bored. His partner was away and he was, like she was, at a loose end.
"He decided to go to Asda, not because of any real burning need for anything but as much to fill in time as anything else.
"And their meeting, like that night, like what followed was unplanned and it was pure chance.
"Vincent left his flat and he was walking towards his car, intending to drive to Asda, when he passed Joanna's kitchen window.
"Her blind was up - it always was. It was broken, her boyfriend confirmed.
"The light in the kitchen was on. Joanna was in there. She looked up and saw Vincent, her neighbour. He noticed her.
"There was a nod and acknowledgement between the two and she indicated or beckoned for him to retrace his steps and to come in."
Mr Clegg told the jury that the invitation marked an "unfortunate starting point for the defence case".
"He went into her flat because she had opened the door and invited him in," the lawyer said.
"He took off his coat and hung it on the coat rack that was in her hall and she offered him a drink.
"He declined because he was driving later.
"They then introduced themselves to each other and chatted, as neighbours would.
"She said that her boyfriend was away and she was alone and he said that his girlfriend was away and he was alone.
"And as the two of them talked inside that flat, Vincent Tabak completely misread the situation that he had walked into.
"Joanna was only being sociable, as many neighbours would be, particularly as it was Christmas.
"He misread her friendliness towards him and made a move towards her as if he was about to kiss her on the lips."
Tabak then put his arm around Miss Yeates.
"She screamed, it was a loud piercing scream," Mr Clegg said. "He panicked. He put her hand over her mouth to stifle the screams.
"He said to her 'Stop screaming'. He apologised and said he was sorry.
"He took his hand away and she carried on screaming. He panicked. He put one hand around her throat and the other over her mouth.
"In seconds - far less than a minute - Joanna went limp. She was dead.
"He never intended to kill her. Nothing had been planned, nothing was premeditated, it was pure chance that he had passed by her kitchen window when she was preparing to start cooking."
Mr Clegg said he would not try to justify Tabak's actions after her death, saying his client was "living a lie" by attending dinner parties and attempting to carry on his life as normal.
He said it was "frankly disgusting" that Tabak had tried to hide the body and "did everything he could to cover his tracks".
Tabak took Miss Yeates's sock and the pizza she had bought that night before putting them in a public dustbin.
He told the jury: "We all know what he should have done. He should have phoned police - he never did.
"That is something that he must bear responsibility for."
The court heard earlier that Tabak researched the unsolved murders of Melanie Hall and Anni Dewani after the killing.
He also looked up satellite imagery of the site where he dumped Miss Yeates's body before she was found by dog walkers on Christmas Day.
The case was adjourned until tomorrow when Tabak - who denies murder but admits manslaughter - will give evidence.