The murder trial for Eddie Ray Routh, the man who killed American Sniper and former Navy SEAL Chris Kyle is now underway, and details from the proceedings revealed that Kyle described his killer as "straight up nuts."

While on the way to the Texas gun range that fateful day last February 2013, Kyle and his friend Chad Littlefield were exchanging texts about Routh, and the two agreed that there is something really wrong about him, the Associated Press reported.

"This dude is straight-up nuts," texted Kyle.

"He's (sitting) right behind me, watch my six," Littlefield texted back, using a military reference for watching one's back.

This is what Routh's lawyer Tim Moore opened in his statement, saying that Routh's insanity was so obvious that Kyle and Littlefield themselves admitted it in their exchange of texts.

Moore used the texts in his defence, saying Routh felt severe mental strain that day and feared that Kyle and Routh would soon turn on him. "He thought he had to take their lives or he was in danger," Moore said.

But a prosecutor said that even with a history of mental illness, Routh still knew the difference between right and wrong. Erath County District Attorney Alan Nash described the Routh as "a troubled young man." He said that on the morning of the killings, he numbed himself with marijuana and whiskey, adding that mental illness should not absolve him for his crime.

"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," Nash said.

For her part, Taya testified that Littlefield and her husband were close, and even enjoyed spending time with veterans as a way to help them move on from the war and get back into civilian life. She recalled how her husband himself struggled with post-traumatic stress disorder, since he was often irritable and slowed by physical ailments.

She added that her husband had been approached by Routh's mother to help her son, who was a small arms technician who served in Iraq and was deployed to earthquake-ravaged Haiti before leaving the Marines in 2010.

Authorities revealed that after the February 2013 shooting, Routh drove to his sister's house in Kyle's truck, admitted to the killings and told his sister "people were sucking his soul."

The case against Routh has received widespread media attention mainly due to the success of Clint Eastwood's movie American Sniper, which is based on Kyle's life. He was played by Bradley Cooper while his wife Taya was played by Sienna Miller. The film has grossed nearly $300 million.

If convicted, Routh will face life in prison without parole.