Rob Knox (right) had a small part in the new Harry Potter film
A man accused of murdering a young actor in south-east London said he "wasn't thinking" when he lashed out with knives, the Old Bailey has heard.
Karl Bishop, 21, of Sidcup, said a "red mist" descended on him because he was beaten up outside Metro bar and got two knives out to "scare" his attackers.
Rob Knox, 18, from Swanley in Kent, appeared in the latest Harry Potter film. He was stabbed five times.
Mr Bishop, from Carlton Road, denies murdering Mr Knox in May last year.
The defendant also denies five charges of wounding with intent to cause GBH.
Mr Bishop told the court: "I didn't care at the time. I wasn't thinking at the time.
"I wouldn't have expected people to attack someone with knives. It's idiotic."
He admitted stabbing the teenager was "over the top" and told the court it was "dangerous" to carry knives and their use was not "not justified".
"I didn't want to hurt anyone," he said.
Replying to Brian Altman QC, prosecuting, Mr Bishop said he was bothered about killing and said he did not want to use the kitchen knives to "teach a lesson".
"I didn't intend to use the knives," Mr Bishop said.
"If you show knives to people they are normally quite scared. I would be.
"I was surrounded. There were bottles coming from all angles, so I took the knives out so they wouldn't come near me."
Replying to Mr Altman's suggestion he should not have retaliated, Mr Bishop said: "If I went to the ground I could have been the one dead."
"[But] course I do [have] big regrets."
The trial continues.