Daniel Fitzsimons, a former paratrooper, claimed diminished responsibility in the deaths of Briton Paul McGuigan and Australian Darren Hoare in August 2009, The Guardian reported.
Fitzsimons cited military animosity with McGuigan, even though he did not know him.
"Paul was with the marines and I was with the parachute regiment, and as everyone knows there is rivalry between the two," he testified. "He started to insult me and insulted two friends of mine who had died in Iraq."
Later, he said, the two men barged into his room and McGuigan took his rifle and pointed it at him.
"He said 'I am going to kill you.' I raised my pistol and shouted to Paul twice to put down his weapon, but he did not respond. Then I made my decision ... as a trained soldier, I shot him twice in the chest and a third time in his face as he fell.
"The Australian then tried to fight me for the pistol. ... I pulled the trigger and put two, maybe three bullets in his chest."