Clive Carter, 35, denies murdering Khanokporn Satjawat at the Clyde Auditorium within the Scottish Exhibition and Conference Centre (SECC) in Glasgow. He is on trial at the High Court in the city.
He has admitted the killing on the grounds of diminished responsibility, but said he had no memory of the attack.
He denied in court yesterday that he orchestrated a cover-up after the death on November 12 last year.
Under cross-examination for a second day yesterday, Carter told advocate depute John Scullion, prosecuting, he did not remember destroying evidence.
Miss Satjawat's skull and most of the bones in her face were smashed in the attack in the ladies' toilets on the auditorium's second floor as she attended an HIV conference.
Carter, of Motherwell, North Lanarkshire, said he remembered arguing with his victim and the next thing he recalled was eating his lunch.
Carter was asked why he had lied to the police by claiming he had seen an Asian man walking about with a fire extinguisher and had taken it from him.
He replied: "At the time I thought I was helping the police."
When asked if he had in fact seen an Asian man, Carter said: "In my head yes."
Mr Scullion said: "Was it because you knew that two delegates had seen you with a fire extinguisher." Mr Carter replied: "I don't know."
He then added: "At the time I didn't know that."
Carter also told the jury he had no recollection of washing blood from the fire extinguisher and then hiding the equipment in a room in the complex.
Mr Scullion said he did not remember taking the extinguisher to a room to clean it of blood. He also denied that he hid his bloodstained G4S blazer.
Jurors also heard how Carter told a psychiatrist: "It was just a silly argument.
"It's ridiculous. I'm in prison for killing a woman over checking a badge."
Carter told the psychiatrist that Miss Satjawat told him to stop pestering him and was annoyed at being asked to show her security pass.
Earlier prosecutor Jon Scullion showed CCTV footage taken 16 minutes before her death which showed Miss Satjawat was wearing her pass round her neck.
He agreed that he beat her with the extinguisher while she was wearing her badge.
The prosecutor added: "Was she wearing her badge when she left the auditorium to go to the toilet," and he replied: "No."
Mr Scullion went on: "Can I just suggest to you that you that you have been lying all day about your memory of this event?" Carter shouted: "No, not at all."
The prosecutor added: "You have known throughout that you beat her to death." Carter again shouted loudly: "No, not at all."
Carter denies murdering Miss Satjawat and attempting to defeat the ends of justice by hiding evidence.
The trial before Lord Matthews continues.