Matthew Daley knifed Donald Lock after the 79-year-old shunted into the back of his vehicle at around 16mph on July 16 last year.
Lewes Crown Court heard today Mr Lock was attacked after he asked ‘why did you break so suddenly’?
Daley 35, of St Elmo Road, Worthing, denies murder on the grounds of diminished responsibility.
Jurors heard his police interview today, conducted two days after the attack.
In the clip Daley said he was ‘sorry for what he had done’.
He said: “There was nothing bothering me that day. I was the happiest man alive. I’d had a really nice day with my friends.
“I’m not happy that the man has died. I’m not happy that in the final moments of his life he was in that much pain and I don’t want to be reminded of it.
“I feel very sorry about what I have done and I don’t want to see anything like that happen in my lifetime again.”
Daley sat in the dock, flanked by two carers from the medium-security unit he has been treated at since his arrest.
As the tape was played to the court he looked down at the ground, only occasionally looking up at the video.
During the interview Daley also stood up and demonstrated to detectives how he stabbed Mr Lock while he was on the ground in the throat, chest and head. After the stabbing Daley also texted partner Lea Luff, who had ended a five-year on-off relationship a week earlier, to give her the registration of another vehicle that was similar to his, so he could hide what he had done.
Daley told officers he put his blood-soaked clothes that he washed in a rubbish bag, before dumping them in a neighbour’s bin.
When police searched his flat, they also found blood on a door handle.
After the attack Daley said he thought about walking to Brighton or Arundel as he was ‘really anxious’ and ‘didn’t know what to do’, so aimlessly walked around Worthing.
He said: “I looked for what I would need if I was going to leave town forever - like my favourite movies, a framed picture of my horse, Honey, and her passport and 14 Leslie Howard movies to keep me entertained at night.
“I thought that by 6am I would want to be gone. My actions were different to the way I was thinking. I was anxious, I wanted to stay with my friends and do the things I’d always been doing.”
But instead of fleeing Daley walked to his friend Katherine Tinkler’s house, before leaving at around 5.30am the next morning.
He then called his mother, Lynda Daley, and asked her if she had heard about the incident on the A24 the previous day.
She kept him on the phone and directed armed officers to his location.
Daley said: “I couldn’t do it on my own, I needed someone with me. I was feeling a bit anxious and just wanted to tell someone. I told my mum ‘I’ve killed someone’ and she kept asking about my health.
“Then I heard sirens coming from where I was in Broadwater.”
The trial continues.