The man accused of murdering a model said he was "totally disgusted" with himself for having sex with her corpse.
Mark Dixie, accused of killing Sally Anne Bowman, said he did not realise she was dead until after he had sex with her body, the Old Bailey heard.
He said he became suspicious when the teenager did not react to bites on her face and body.
Mr Dixie, 37, denies murdering the 18-year-old in Croydon, south London, during a sex attack, in September 2005.
Giving evidence, he said: "I don't know what went through my mind. I took full advantage of someone and I should not have done it."
'Pair of legs'
He told the Old Bailey: "I am totally disgusted with myself."
He said he had been biting her and there was no reaction.
"I pulled the hair away from her face," he said.
Prosecutor Brian Altman asked: "You killed her for sex, didn't you?"
Sally Anne Bowman was stabbed seven times
Mr Dixie replied: "No. I did not go out looking for someone to kill for sex."
He also denied stealing Miss Bowman's underwear as a trophy.
The court heard Miss Bowman was found in a pool of blood in the driveway of her home in Blenheim Crescent.
Prosecutors allege Mr Dixie stabbed her seven times and raped her as she was dead or lay dying.
But Mr Dixie has told jurors that he found Miss Bowman's body after a drink and drugs binge.
He told the court: "I have gone over it during the last three years. I don't know what went through my mind.
"All I saw was a pair of legs... shirt put down to the waist and I took advantage of her.
Anthony Glass, QC, defending, asked if he thought she was dead or unconscious.
Mr Dixie said: "I would not have expected to see anyone dead in that street.
"I thought she might have passed out or fallen over."
But he admitted there was no reaction during the sex attack, even when he bit Miss Bowman.
Mr Glass asked Mr Dixie, "Did you, in fact, realise she was dead?"
To which he responded: "After a while. I panicked. I stood up. I looked at her. I did not know what to do."
Mr Dixie described how he then placed cement dust and concrete from a nearby skip on Miss Bowman's body to conceal his DNA.
The trial continues.