Mark Paul was asked by Christopher Wilson-Smith QC, his defence counsel, if he intended to kill or seriously injure his victim.
Mr Paul sobbed, wiped tears from his eyes and said: 'No, I did not. I thought he would have a headache.'
The skull of John Lavender, a 28-year-old insurance agent, was cracked into four pieces by the two-handed blow to the back of his head.
He died from severe brain injuries 12 hours later, the Old Bailey was told.
Mr Paul, 20, a jewellery salesman of Battersea, south London, denies murdering Mr Lavender, who also lived in Battersea.
Mr Paul told the court that he did not realise he had inflicted serious injuries and when he learnt that Mr Lavender was dead he could not believe it.
He denied prosecution claims that he had used the bat in a baseball game with friends a few days later.
He said he had sat under a tree and watched the game. Breaking down again, he sobbed: 'I didn't join in because of the bat - I didn't want to touch it.'
He explained that on the evening of the killing, almost a year ago, he had drunk five or six pints of lager and smoked two or three cannabis joints.
When he reached home, he and his flatmate Andrew Christie went out with the baseball bat. 'I was excited, I was tipsy and in high spirits,' he said.
The two men planned to cause criminal damage to cars but not people, Mr Paul said.
He told the jury that Mr Christie had smashed one car windscreen and they were on their way home when they spotted Mr Lavender.
Martin Heslop, for the prosecution, told the jury that the issue they must consider would be Mr Paul's intent when he struck the fatal blow.
Under cross-examination later, Mr Paul admitted that he had hit Mr Lavender to 'show off'.
He also agreed that he had run at Mr Lavender and 'quite deliberately' struck him on the back of the head.
Showing how he held the heavy bat in a two-handed grip, Mr Paul claimed he had not used a full swing to strike the blow.
He had not called for help, nor made an anonymous call for an ambulance because he did not realise Mr Lavender was seriously hurt, even though Mr Christie had been very concerned about his condition.
The trial continues today.Reuse content