A teenager accused of using a stolen vehicle as a "weapon" to murder a Wirral policeman had been using cannabis since the age of six, a court heard.
Clayton Williams, 19, from Wallasey, had been buying drugs and had smoked a "spliff" before stealing a Mitsubishi pick-up truck in a burglary and mowing down PC Dave Phillips.
He claimed the drugs may have slowed his reactions when he ran over the 34-year-old officer, during an 80mph police chase through Wallasey, in the early hours of October 5, last year.
Father-of-two PC Phillips had been deploying a stop-stick or stinger device to puncture the Mitsubishi's tyres, but stood "no chance" when the vehicle hit him, killing him instantly, Manchester Crown Court heard.
But Williams, who denies murder, maintains he did not intend to injure or kill PC Phillips and told police he felt "sick" when he saw on the news the officer had died.
Before he was arrested next day he burned his clothes and disposed of the ashes in bushes along the River Mersey.
Hours later when officers arrested him he said: "Going down for life me, kid."
Today the jury heard what Williams told officers after his arrest.
He said he burgled a shop, Oxton Estates in Birkenhead, which he came across while walking back from getting "some weed" and saw the keys to the Mitsubushi on a desk so stole the vehicle too.
In the first of three prepared statements he said he did not see PC Phillips until he was "almost on the spikes".
He continued: "I thought and knew I had injured him. I did not realise I had killed him. I saw it on the news and I felt sick.
"I felt devastated for what I had done to the officer and great pain and suffering I have caused his family.
"I did not intend to kill PC Phillips. I fully accept I am responsible for his death to the manner of my driving.
"I know I can't bring him back but I hope my admissions will prevent putting PC Phillips's family through the ordeal of a trial."
Williams told police he suffered "bad memory loss" and "smoked a lot of weed".
In further police interviews, where he answered "no comment" to many questions, Williams said the drugs in his system meant "my reactions might have been slower" when he struck the officer in the truck.
But he maintained he did not intend to harm Pc Phillips or Pc Thomas Birkett, 23, who had to dive out of the way after the Mitsubishi hit his colleague.
Williams denies a second count of attempted grievous bodily harm with intent to PC Birkett.
He has admitted the burglary where the car was stolen and aggravated vehicle taking.
A second man, Philip Stuart, 30, the passenger in the car with Williams, has admitted burglary and aggravated vehicle-taking, by being allowed to be carried in the Mitsubishi.
PC Phillips widow Jen and extended family sat in the public gallery as the final moments of her husband's life was again shown to the jury on video recorded by a police car pursuing the Mitsubishi.
Earlier pathologist Dr Brian Rodgers said PC Phillips was most likely struck when he was standing or moving upright from a crouching position as he laid the stinger device over the road.
He said the cause of death was shock and haemorrhage after the officer's aorta was severed in two, though there were a number of other injuries to his body.
Dr Rodgers said the main injury was "unsurvivable" and he would have died "pretty much instantly" after being hit by the truck.
The trial continues.