Eddie Ray Routh has been found guilty of murdering American Sniper author Chris Kyle and another man at a Texas shooting range.
Jurors returned the verdict against the former Marine, whose lawyers had mounted an insanity defence and argued he suffered from psychosis.
The 27-year-old will receive an automatic life sentence without parole for the deaths of the Navy SEAL and his friend Chad Littlefield.
They had taken Routh out shooting at the Rough Creek Lodge and Resort on 2 February 2013 after the defendant’s mother asked Mr Kyle to help him.
Family members say Routh suffered from post-traumatic stress disorder after serving in Iraq and Haiti after the 2010 earthquake.
Dr Randall Price, a forensic psychologist, testified during the trial that Routh was not legally insane, and said he may have got some of his ideas from watching television.
He said Routh had a paranoid disorder which was exacerbated by his use of alcohol and marijuana.
Defence lawyers said that Mr Kyle described Routh as “straight-up nuts” in a text message to his friend as they drove to the shooting range.
They added that the defendant, who had been prescribed anti-psychotic medication used to treat schizophrenia, believed the men planned to kill him.
In a phone call between Routh and a reporter from The New Yorker magazine, he said he became annoyed because Mr Littlefield was not shooting, but appeared to be watching him instead.
During the call, he said: “Are you gonna shoot? Are you gonna shoot? It’s a shooting sport. You shoot. That’s what got me all riled up.
“I had to take care of business. I took care of business, and then I got in the truck and left.”
The bodies of the two men were found at around 5pm. Both had been shot several times.