West Mercia Police Constable Benjamin Monk gave evidence for the first time in his defence at Birmingham Crown on June 2, after he was charged for the murder of Dalian Atkinson.
Prosecutors claim West Mercia Police Constable Benjamin Monk, who denies murder and manslaughter, used unlawful and unreasonable force during a final 33-second firing of the Taser, and by then kicking the former sportsman in the head.
Jurors heard that he used three Taser cartridges before Mr Atkinson was handcuffed near his father's home in Meadow Close, Telford, Shropshire, in the early hours of August 15 2016.
Mr Atkinson died 70 minutes after the incident in hospital.
PC Monk said he felt "a big relief" when Mr Atkinson was floored after being Tasered the third time, as he said the two previous Taser strikes had been ineffective.
The 43-year-old said: "He (Mr Atkinson) was very, very scary."
"And the device which I thought might work for me, hadn't worked and I was terrified."
It was reported that Mr Atkinson had smashed the glass in the front door of the property belonging to his father.
The two officers, PC Monk and Bettley-Smith, had been sent to the address, after members of the public called 999, reporting a concern for the welfare of the elderly occupant.
During the course of the first two failed Taser strikes, Bettley-Smith called for back-up, activating her emergency button, calling all available units to the location.
However PC Monk told jurors that one of the back-units reported they were still "sixty minutes away".
He said: "That is a transmission I definitely remember, because it struck absolute fear in me to think the nearest crew was so far away."
PC Monk said, having backed away into the street, he tasered Mr Atkinson a third time, and this time the ex-football star "stopped moving towards me and seemed to stop where he was".
He added: "If this cartridge didn't work, Ellie (Bettley-Smith), me, the gentleman inside the house, were potentially done for - so the fact it worked it was a big relief."
The officer told jurors the force he used to kick the former sportsman as he lay on the ground was "about a four" out of 10 on a sliding scale.
The court has already heard evidence that PC Monk's boot connected with Mr Atkinson's head, but the officer said that at the time he believed he had kicked the 48-year-old "in the shoulder".
PC Monk said: "There wasn't an intention to kick him anywhere, it was an instinctive act, a desperate, instinctive act because this was the last thing had."
He officer told the court that he "wanted Mr Atkinson to stop getting up because I thought if he's going to get up, we were going to die."
The court has previously heard that Taser-trained PC Monk had depressed the weapon's trigger for 33 seconds, during the third strike.
When asked if he had been conscious of doing so in evidence, he replied:
"Absolutely not. Absolutely not."
He told jurors that when he found out the news from an inspector back at the police station, it left him "devastated".
The constable said: "I had no idea it was going to end the way it did, I never wanted it to end the way it did."
PC Monk is alleged to have intended to cause really serious injury to Mr Atkinson after those two initial uses of the Taser proved ineffective.
Bettley-Smith, 31, is also on trial, and denies a charge of assault occasioning actual bodily harm, by using her force-issue extendable baton.
The trial continues.