Killer Mark Hennessy was agitated and looked "like a man possessed" in the moments before he was shot inside a car.
The 40-year-old, who abducted and killed Jastine Valdez, was shot by 'Garda A', whose evidence was heard at an inquest into his death yesterday.
The experienced detective told the inquest he was left with "no option" as he believed that Ms Valdez was in the car and in imminent danger.
He told the Dublin Coroner's Court that he had not been on official duty on May 20, 2018, but was catching up on paperwork and said he would be available if needed to help with the search for Ms Valdez.
The 24-year-old Filipino student had been abducted on a busy road in Enniskerry, Co Wicklow, the previous day.
Garda A was conducting searches around Shankill and Dun Laoghaire for the 17D Nissan Qashqai that Hennessy was driving.
From a radio call, Garda A was informed that a member of the public was following Hennessy and there was a second person in the car.
Hennessy drove into Cherrywood Business Park, and Garda A followed him and stopped in front of the Qashqai.
"He was in the driver seat, he was moving from side to side. He looked uneasy and agitated," Garda A told the court.
"I blocked the Nissan to the front and another unmarked car blocked it to the rear. I put out an urgent assistance request.
"Mark Hennessy was moving around in the front of the vehicle in a very agitated state. He was struggling with something in the front passenger step well area.
"I shouted at him 'Armed police, drop the f**king knife'. I shouted this a number of times.
"I took up position and began drawing my weapon from the holster because of the seriousness of the situation. I was concentrated on the blade exposed."
Garda A said he was satisfied Ms Valdez was in the car given the information received and his assessment of the situation.
He said a colleague, 'Garda C', had attempted to open the door of the car, but it was locked.
"With the information I had, I was in great fear that Jastine Valdez was in that vehicle," Garda A told the inquest.
"I observed the knife moving from a higher position and saw it was being lowered into skin. I thought he was slitting Jastine Valdez's throat.
"I had to take decisive action. I shouted: 'He's going to slit her throat'. In that split second, I thought the threat to her life was imminent."
Garda A said Hennessy had been non-compliant with all demands and directions. He said he "took aim at his chest and discharged one bullet."
After assessing that he was of no further threat, he searched the car for Ms Valdez.
He said his "stomach dropped" and his heart sank when he realised she wasn't in the vehicle.
The detective told the coroners court: "I had no other option but to do what I did based on what was unfolding in front of me."
He added that he had been concentrating on "the track of the knife".
"My aim was to rescue Jastine Valdez and arrest Mark Hennessy," he said.
He said that from what he had seen, it was "absolutely necessary" to discharge the firearm.
Hennessy was pronounced dead at the scene at 8.38pm.
The inquest heard pathology evidence from Acting State Pathologist Dr Linda Mulligan, who gave the cause of death as a gunshot wound to the right upper arm and chest.
Hennessy was deemed to have been mildly intoxicated at the time of his death and there was evidence of recent cocaine use, according to the autopsy.
The inquest continues today.