"Tori's was a preventable death." Five years on, the Lindt cafe sniper is overcome with guilt.

When Mark Davidson pulls the trigger he has pin point accuracy.

As the police sniper explained to Liz Hayes in an interview with 60 Minutes, if he’s shooting a person front on, he’s able to hit the tiny bit of skin between the lip and the nose.

He’s deadly. A trained killer.

Here’s part of the interview with 60 Minutes. Post continues after video.

In December 2014 in Sydney’s CBD, Mark spent 16 hours posted outside the Lindt Cafe. His eye watched the terrorist through the barrel of a gun for the entire ordeal – as 18 terrified people were held hostage long into the night.

Two people died that day; cafe manager Tori Johnson and barrister Katrina Dawson, and five years on Mark admits he could have shot and killed Man Haron Monis, but he didn’t pull the trigger.

“In my opinion, I believe Tori was a preventable death,” Mark told 60 Minutes. “I’ve lived with that for years.”

Mark believes his team - as a collective - didn't play how they train. "We could've saved the hostages. We could have saved Tori at least," he said.

“Things occurred on that day I don’t understand.”

Ten hours into the stand-off, Mark says he had a clear view of Monis - who had told the hostages he had a bomb - for 10 whole minutes through one of the cafe's windows.

But in order to justify shooting him, Mark believed he needed to know whether Monis was pointing a shotgun at hostages and whether he had a device that could set off a bomb in his hands.

When he asked police command about that information, he was told it wasn't available.

"You're weighing up shooting someone, and if you think you're not justified, you're facing murder charges yourself," he told 60 Minutes.

"There was some doubt in my mind as to whether I was justified in shooting him ... so I didn't."

The threat of a bomb was one of the biggest reasons Mark and his team, who were stationed in various buildings surrounding the cafe, didn't act.

Had they been given access to some of the escaped hostages and their information, they would have been able to piece together vital information.

By asking questions about the backpack Monis was carrying where he claimed to be carrying a bomb - its weight, any visible wires, how it was being handled - they could have ruled out a bomb completely.

But the hostage interviews, which included several accounts of how "there were no wires" and "the bag was light, there was no sagging from weight" never made it back to Mark's team.

The other crucial information that never made it to Mark was a text message from Tori Johnson that said "he's sitting in the corner all by himself."

He was referring to the 10 minute window that Mark talks about - the clear shot he never took. Mark says getting the information from that text message would have helped to mitigate any doubt.

Hours later, Mark watched Tori get on his knees and put his hands behind his head. He urgently got that information back to the command post, but it never got through to the right people.

He watched Tori's execution.

Mark has spent the past five years believing Tori's death at least was preventable. He still visits the Lindt Cafe regularly, and sits at table 9 where Tori lost his life.

"I can't explain why...maybe to pay homage or respects," he said, adding that he gets flashbacks of Tori's last moments.

An independent review into the actions of police found that officers in the field were always at a disadvantage because of a lack of a clear plan of action.

NSW police says Mark had been stationed inside the Westpac Building and that his rifle could not have penetrated both the window glass in that building and the window glass at the cafe.

Breaking the Westpac glass in order to shoot through it would have taken significant time and been noisy.

That means it could have been heard by Monis, who may have retaliated by detonating the bomb believed to be present at the time, the force said in a statement.

"The coroner concluded that the decision not to fire was completely reasonable," NSW police said.

Mark disagrees. He thinks he could have made that shot and killed Man Monis immediately.

"There was an achievable option. It's upsetting...that's what we're there for," he told Liz Hayes.

The former police sniper has been medically discharged from the force with PTSD, and still carries the guilt and pain from that day heavy in his heart.

With AAP.