A former Marine veteran has been found guilty of fatally shooting Navy SEAL sniper Chris Kyle and his friend Chad Littlefield at a shooting range near Chalk Mountain, Texas, two years ago.

Eddie Ray Routh was convicted of capital murder after a Texas jury, made up of 10 women and two men, took less than two hours to decide on a guilty verdict on Tuesday, the Associated Press reported. The 27-year-old, who struggled with Post-Traumatic Stress Disorder after serving as a small arms technician in Iraq before leaving the Marines in 2010, had been pleading not guilty by reason of insanity.

Kyle, who has been credited with being the U.S. military's most lethal sniper, was fatally gunned down along with friend Littlefield by Routh at a Texas shooting range on February 2, 2013, Reuters reported. Kyle had taken Routh to target practice after Routh's mother requested the 38-year-old sniper to help Routh deal with PTSD. But once at the gun range, Routh killed Kyle.

After a 9-day trial, State District Judge Jason Cashon sentenced Routh to life in prison without the possibility of parole on late Tuesday.

"We have waited two years for God to get justice on behalf of our son," Littlefield's mother, Judy, told reporters outside the courthouse. "And as always, God has proven to be faithful."

"We're so thrilled that we have the verdict that we have tonight," she added, thanking the reporters "for being so compassionate."

However Routh's defense team said they would appeal the conviction, arguing that he suffers from psychotic episodes caused by PTSD and other factors, according to ABC News.

Prior to Kyle's murder at Rough Creek Ranch, Routh had been admitted to inpatient psychiatric treatment. In the past five months, he had also been taken to a mental hospital, with police records stating that he was suffering from PTSD.

"He didn't kill those men because of who he wanted to be, he killed those men because he had a delusion," Warren St. John said. "He thought that they were going to kill him."

But on Tuesday, prosecutors said that whatever episodes Routh suffers are self-induced through alcohol and marijuana abuse. "That is not insanity. That is just cold, calculated capital murder. (Routh) is guilty of capital murder and he was not by any means insane," Erath County assistant District Attorney Jane Starnes said, citing Routh's apology to Kyle's family as further evidence of a guilty mind.

Meanwhile under Texas law, even if a person was suffering from a mental illness, they can be found guilty as long as they understood that what they did was wrong, Fox News reported.