A neighbour killed Jo Yeates after she screamed when he went to kiss her, a court has heard.
Vincent Tabak had "completely misread" the 25-year-old's friendliness when he was invited into her flat, his QC William Clegg told Bristol Crown Court.
When she screamed he panicked and covered her mouth and held her neck until she "went limp", the court heard.
Tabak, 33, has admitted Miss Yeates's manslaughter on 17 December last year but denies her murder.
Miss Yeates, originally from Ampfield, Hampshire, was strangled at her flat in Clifton, Bristol. Her body was found on Christmas Day.
Mr Clegg said Tabak was invited into Miss Yeates's flat when she saw him walking past her kitchen window.
It was here, Mr Clegg said, where the pair introduced themselves to one another "and chatted as neighbours would".
"Vincent Tabak misread the situation that he had walked into. Joanna was only being sociable as many neighbours would," Mr Clegg told the jury.
"But he misread her friendliness towards him and made a move towards her as if he was about to kiss her on the lips."
Tabak's girlfriend had been at a Christmas party that night while Miss Yeates's partner was away with family in Sheffield.
Tabak claimed he was in the flat for about 10 minutes before Miss Yeates screamed.
But the defence also claimed screams heard by nearby residents could also have been students celebrating Christmas.
"What Vincent Tabak did was a dreadful crime - but it wasn't something he had planned, intended or wanted to do," Mr Clegg said.
"He panicked and in literally a few seconds he discovered to his horror that she had died."
The defence QC said counsel would not try to excuse Tabak's "disgusting" conduct after her death as he tried to cover his tracks.
But he claimed that Tabak's "calculating" behaviour afterwards did not mean he intended to kill Miss Yeates.
Turned off oven
"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."
Mr Clegg said that after killing Miss Yeates, Tabak switched off the oven Miss Yeates had turned on in preparation for baking.
He took a Tesco pizza that Miss Yeates had bought on the way home, carried her body to his flat and put it in a bicycle cover.
He moved his car to the left of the house and placed the body in the boot, where it remained while he went to the Asda supermarket in Bedminster, Mr Clegg said.
"Why on earth he went to Asda, I'll have to wait and see whether you can find any sense in that at all," Mr Clegg said.
"It doesn't look as though it was a particularly rational thing to do."
Mr Clegg said Tabak's driving had been "rather aimless" because he was trying to work out what to do.
Mr Clegg said Tabak had thrown away the pizza, as well as a sock missing from her body, into an industrial bin and "started living a lie".
Earlier, as the prosecution case ended, the jury was told how Tabak searched the internet for details of the maximum sentence for manslaughter.
The day after Miss Yeates went missing Tabak also searched for maps of Longwood Lane where her body was later found.
Computer expert Lyndsey Farmery told the court that Tabak also read news articles about her disappearance.
The jury was told that Tabak looked at Wikipedia pages defining murder and manslaughter in English law before the landscape architect's body was found.
He also looked up definitions of "sexual assault" and "sexual conduct" and read articles on the BBC news website about Miss Yeates's disappearance.
Tabak also researched DNA evidence, forensics and fingerprinting on 22 December, the court heard.
His internet history showed he searched for "CCTV Canynge Road" and used Google Street View to see the area.
Ms Farmery also told the court that Tabak searched for "Joanna Yeates pizza" in reference to the missing pizza.
The trial was adjourned until Thursday when the jury was told Vincent Tabak would give evidence.