"The officer would have been doing a continual threat assessment," Mr McCormack said.
"This officer looked at what was going on and determined he could handle it the way that he did. People are right — this guy is a hero.
"I think the full impact of how horrible this whole thing is just setting in on our people now," he said.
"Like, this is crazy, this is insane, we've never seen this."
The rampage began when a white van careered down Yonge St, Toronto, Canada, just before 1.30pm on a "sunny spring afternoon". Witnesses claim the vehicle was moving fast and appearing to act deliberately, leaping the kerb and speeding down the footpath.
"He started going down on the sidewalk and crumbling down people one by one," witness Alex Shaker told CTV.
"He just destroyed so many people's lives. Every single thing that got in his way."
One man who was at the scene, identified only as Nick, said he "witnessed a few people pass away" in front of him. After mowing down pedestrians, the driver in the battered van sped off.
Toronto police caught the suspected driver, Alex Minassian, 25, who is alleged to be responsible for ploughing into people on the crowded city street, leaving 10 dead and 16 injured.
But the ultimate take-down of the suspect, captured on Instagram by a witness just metres from where the driver had stopped on a footpath, is described by commentators as "the one with the cop — the cop who didn't shoot".
The video begins with the siren wailing and a police officer just 10m away, huddled behind a car, gun drawn, yelling.
Suddenly, the van door opens and the driver appears. A man dressed in black, pointing what appears to be a gun at the officer. The two are yelling at each other but the sound is muffled by the siren — except for the words "get down".
The driver keeps his weapon pointed, quick-drawing the gun like a gunslinger, as the officer turns the siren off. He keeps yelling, "C'mon get down."
Then, unexpectedly, the driver responds with, "Kill me."
But the policeman won't fire.
"No, get down."
"I have a gun in my pocket," the driver says, urging the officer to "shoot me in the head" as he walks towards him, gun pointed. It's tense viewing.
"I don't care! Get down!" the officer says.
In a second video, the scene is captured from a window above. As the driver walks towards the officer, arm extended, the officer stays on him, despite the immediate threat. He keeps advancing, until the driver appears to hesitate. His hands go up, he drops what appears to be a gun, and he finally succumbs.
The officer yells again: "Hands behind your back." He's kneeling on the driver, laying face down on the pavement.
In the most tense 37 seconds, not a single bullet was shot.
"I am paid to explain things and sound confident doing so. But I honestly don't know what to make of this terrifying, remarkable moment," wrote Scott Gilmore, editor-at-large of Canada's national current affairs and news magazine Maclean's Magazine.
"A man may have just killed many people. He rushed out of his van, which could have been a bomb. He pointed what looked like a weapon. And yet this police officer did not shoot.
"At any point if the cop had fired and killed the suspect, the public, his peers, the press, even the driver himself, everyone would have understood. In fact, we likely would have called him a hero.
"What held his finger? Bravery? Training? Compassion? Perhaps we will find out in the days ahead. But whatever it is, it deserves attention."
Mr McCormack said the officer, who was in his 30s, told him afterwards: "I was just doing my job, I wanted to arrest this guy."
"He's shaken up by the whole thing, and shaken up by the magnitude.
"He's been confronted by someone who is making like he has a weapon, threatening the officer's life, trying to get the officer to shoot him, and he arrests that guy."
According to Macleans, the video was captured by a tow truck driver identified as Mike who witnessed the arrest while his truck had broken down at an intersection.
"Mike says the suspect van was driving down the sidewalk and a police cruiser approached from the opposite direction," the publication reported.
"The police cruiser then did a U-turn and began driving 'side-by-side' with the van.
"Mike says the vehicles came to a stop and the officer was first to exit his vehicle with his gun drawn. The suspect then flung open the van door and began pointing a black object at the officer."
The officer has not been identified by name, but commentators online are using the video as an example to American officers on what do to in crisis situations.
Toronto Police Chief Mark Saunders said the officer's behaviour was the epitome of Canada's "high calibre of training that takes place.
"They are taught to use as little force as possible in any given situation," he said.
