Det Garda William Sisk was one of the three ERU officers who were on the road when Mr Carthy emerged from his home carrying a loaded shotgun.
Garda Sisk was standing in between Det Garda Aidan McCabe and Det Sgt Michael Jackson when they shot Mr Carthy. Garda Sisk raised his weapon but did not fire.
Garda Sisk was armed with a high-velocity rifle which fired bullets at 2,000 miles an hour, he told the tribunal. If he had fired a shot from where he was positioned, it could have gone "straight through" Mr Carthy and hit someone else, he said.
"I was concerned that the bullet would enter John Carthy's body and be deflected off muscle tissue or bone and come a different direction out of John Carthy and perhaps go into the village."
However, he said his main concern was for the "uniformed men and the plain-clothes men at the command post".
Garda Sisk said he moved to the left, so that if he took a shot at Mr Carthy it would go towards an empty field, but before he was in position, Sgt Jackson discharged his first shot.
Counsel for the tribunal, Mr Michael McGrath, asked if Garda Sisk felt at risk from people at the command post firing shots.
Garda Sisk said he was the only one with a high-velocity rifle and did not think he was in range of the command post. "They were using different weapons and there was a huge probability that a bullet would have stopped if it hit John Carthy."
He said he did not see and was not subsequently made aware of any local armed garda having fired shots, but "hoped they had their weapons out and ready".
He perceived no danger of crossfire if a local garda had decided that he was going to shoot Mr Carthy.
Garda Sisk said he was not concerned with Mr Carthy's motivation when he walked out of his house. "Our training deals with the basic fact that John Carthy had a loaded firearm and was walking toward the village and people's lives were in danger."