Eddie Ray Routh has been found guilty and sentenced to life in prison without parole for shooting and killing Chris Kyle, the soldier celebrated in the film American Sniper, and his friend Chad Littlefield.

The case went to the jury on Tuesday after a prosecutor said in his closing arguments that Routh, 27, acted coldly and deliberately in a deadly ambush of the pair at a Texas gun range in February 2013.

Only two and a half hours later, Routh was back in the courtroom, standing motionless as district judge Jason Cashon informed him that after their deliberations – including eating dinner and picking a foreperson – the jury of 10 women and two men had unanimously found him guilty of capital murder for killing the subject of the blockbuster film American Sniper and his friend on a rural shooting range a little over two years ago.



Silence and stillness prevailed as the verdict was read out, followed by tears and hugs as Routh was escorted from the Texas courtroom to begin his life sentence.



Facebook Twitter Pinterest Chris Kyle, author of American Sniper and subject of a film of the same name. Photograph: Paul Moseley/AP

Kyle’s widow, Taya, had been in court daily and was the first witness called by the prosecution 13 days ago, but was not present to hear the verdict. She had walked out during the defence’s closing statement on Tuesday afternoon, seemingly upset by their line of argument.



But many of Littlefield’s relatives remained and two gave forceful victim impact statements, speaking directly to Routh, who mostly sat still and looked straight ahead towards the judge rather than turn to meet their gaze.



“You took the lives of two heroes; men that tried to be a friend to you. You became an American disgrace,” said Jerry Richardson, Chad’s step-brother. Their words and the jury’s exit promoted emotional scenes inside and around the small courtroom in this town about 100 miles south-west of Dallas.



Outside the building Chad’s mother, Judy, told reporters that the family had “waited two years for God to get justice for us on behalf of our son and, as always, God has proved to be faithful. We’re so thrilled that we have the verdict that we have tonight.”



A former navy Seal reputed to be the deadliest sniper in US military history, Kyle was the subject of the film directed by Clint Eastwood. It has made over $400m globally at the box office and was nominated for six Academy Awards, winning best sound editing at Sunday’s ceremony. The movie, and his bestselling 2012 autobiography on which the film was based, made Kyle an emblem of patriotic heroism for many Americans and an icon for a slew of conservative commentators.



Routh’s legal team did not dispute he carried out the killings but hoped to convince the jury to return a verdict of not guilty by reason of insanity and produced examples of his strange behaviour. They argued he was a paranoid schizophrenic man who was having a psychotic episode when he shot and killed Littlefield, 35, and Kyle, 38, on 2 February 2013.



Facebook Twitter Pinterest Chad Littlefield was killed when he and Chris Kyle accompanied Routh to a shooting range. Photograph: Facebook

While the trial took place amid national focus on the case and the film, there were times when proceedings adopted a distinctively Texan, and sometimes bizarre, tenor.



Prosecutor Jane Starnes told the jury that claims Routh believed in killer “pig people” were “a load of hogwash”. Addressing the jury, her colleague, district attorney Alan Nash, concluded with a plea to the jury’s local pride: “This defendant gunned down two men in cold blood – shot them in the back in our county. Find him guilty.”



Routh’s threatening behaviour at a family fish fry was analysed and even seemingly banal actions were probed as possible evidence of his mental state. Did Kyle and Littlefield breed a seething resentment by forcing their guest to eat lunch at the Lone Star State’s most beloved fast-food chain, Whataburger, when he was not hungry?



Why did he stop to buy a Dr Pepper and burritos in the truck he stole from Kyle? Routh’s defence team mockingly summarised one of the prosecution’s arguments as “he couldn’t possibly be insane because he could negotiate the drive-thru at Taco Bell”.



The case was the highest-profile insanity plea in Texas since the 2006 retrial of Andrea Yates, who was found not guilty after a jury decided she was suffering from postpartum psychosis when she drowned her five children in a bathtub.



The defence had attempted to have proceedings moved from Erath county, where Kyle was killed, on the basis it would be impossible to get a fair trial. On the second anniversary of his death – only a couple of days before jury selection began with candidates being asked whether they had seen the film or read the book – the Republican governor of Texas, Greg Abbott, officially declared 2 February to be Chris Kyle Day.



Legal experts had cautioned that the defence faced an uphill task because Texas’s definition of insanity is narrow and allows for even people with significant mental problems to be found guilty provided they are judged to have known right from wrong at the time of the crime.



Capital murder in Texas carries a mandatory sentence of life without parole – the prosecution opted not to seek the death penalty. In line with state law the jury was not allowed to consider the potential consequences for Routh of a verdict of not guilty by reason of insanity. The defence was not permitted to speculate that such a verdict might see him spend his life in a secure psychiatric facility, as opposed to being released back into society.



Each side produced medical experts who contradicted each other’s testimony as to the severity and origins of Routh’s unstable behaviour. The prosecution argued that his actions were the result of “mood disorders” spurred by heavy use of alcohol and marijuana and that his mental illness claims were exaggerated.



The defence called a psychiatrist, Mitchell Dunn, who testified that he had interviewed Routh at length and found that he had delusions about “cannibal” work colleagues and killer pigs: “He began to think that Mr Kyle and Mr Littlefield were some type of pig assassins – hybrid pigs sent here to kill people.”



In the month before the killings Routh kidnapped and threatened his girlfriend, yet proposed to her the day before he met Kyle and Littlefield. Family members testified about the former marine’s descent into mental illness since he left the military in 2010, saying they believed he had post-traumatic stress disorder, though as a weapons technician he did not see direct combat while stationed in Iraq and Haiti.



Routh did not testify during the trial but his court-appointed lawyers claimed that at the rifle range he came to believe Kyle and Littlefield were plotting to kill him, so he had to shoot them first, in a misguided form of self-defence. The jury heard that Kyle himself had sent a text message to Littlefield as the three drove to the range describing Routh as “straight-up nuts”.



However the prosecution argued that Routh’s actions after the murders – he confessed to his family, tried to evade arrest and told an officer during questioning that he had made a mistake – showed he knew right from wrong. They suggested he was jealous of Kyle and murdered him and Littlefield in a fit of pique because they behaved coldly towards him on what was supposed to be a therapeutic afternoon outing to the range.



Gene Cole, an officer at the Erath county jail, where Routh had been held since his arrest, said the prisoner told him that he “shot them because they wouldn’t talk to me. I was just riding in the back seat of the truck and nobody would talk to me.”



As for the “pig assassins”, one prosecution witness said Routh could have borrowed the idea from a scene in an episode of Seinfeld or a reality TV show called Boss Hog.



Alan Nash, the prosecutor, told the jury that “the evidence will show that mental illnesses, even the ones that this defendant may or may not have, don’t deprive people from being good citizens, to know right from wrong”.



On Tuesday night the jury quickly agreed. Routh has the right to appeal.