A videotaped confession by the former Marine accused in the fatal shootings of "American Sniper" author Chris Kyle and his friend was shown to jurors Monday as the trial resumed in Texas.

Eddie Ray Routh, 27, confessed to the February 2013 slayings but has pleaded not guilty by reason of insanity.

"I'm just sorry for what I've done," Routh said in the interview with Texas Ranger Danny Briley, according to The Dallas Morning News.

"I've got tons of people eating on my soul right now," he added.

Criminal law experts say the case hinges on whether the defense can prove Routh was insane at the time and did not know the killings at a gun range constituted a crime. 

"He stated that he knew it was wrong to kill them, that he wished he hadn't done it," Briley said under questioning by the prosecution.

Kyle served four tours in Iraq and made more than 300 kills as a sniper for SEAL Team 3, according to his own count. He earned two Silver Stars for valor. After leaving the military, he volunteered with veterans facing mental health problems, often taking them shooting. He took Routh to the shooting range at the request of the troubled veteran's mother.

The case has drawn intense interest, partly because of Kyle's memoir. An Oscar-nominated film based on the book has grossed more than $300 million in North America.

Family members say Routh suffered from post-traumatic stress disorder after serving in Iraq and Haiti after the devastating 2010 earthquake. Defense attorneys say Routh, who was taking anti-psychotic medication, was insane when Kyle and his friend, Chad Littlefield, took the former Marine to the shooting range on Feb. 2, 2013 to provide support and camaraderie. Routh, his lawyers say, believed the men planned to kill him.

In the confession played Monday, Routh said of Kyle, "If I did not take down his soul, he was going to take down mine," The Dallas Morning News reported.

Prosecutors say Routh was a troubled drug user who knew right from wrong, even with a history of mental illness. They filed documents saying Routh smoked marijuana, drank excessively and had a history of killing small animals. On the day of the killings, Routh had been drinking and smoking marijuana and threatened his girlfriend with a knife, one of the documents says.

A Texas Ranger testified that authorities found marijuana, a near-empty bottle of whiskey and medication for schizophrenia while searching Routh's small wood-framed home after the shooting.

Some of Routh's psychiatrists at Green Oaks Hospital, where he was admitted in September 2012 and in January 2013, are expected to testify. The testimony could show that Routh deliberately put himself into a violent state, said Park Dietz, a forensic psychiatrist who examined Andrea Yates, who was found not guilty in 2006 by reason of insanity in the drowning deaths of her five children.

"Voluntarily induced intoxication is not an excuse for the mentally ill," he said.

On Thursday, prosecutors presented a video in which officers spoke with Routh as he sat in Kyle's pickup. He refused to leave the vehicle and eventually sped off, with police in pursuit. The video shows a police vehicle ramming the pickup, which became disabled along the side of the road.

Last week, a former deputy testified that he overheard Routh after he'd been taken into custody say he shot the men because they wouldn't talk to him as the three drove together to the shooting range.

Kyle's widow, Taya Kyle, testified about her husband's passion for helping veterans and gun safety. Sometimes choking up and wiping away tears, Kyle testified that her husband and Littlefield were close, and enjoyed spending time with veterans as they eased back into civilian life.

Jurors have three options: find Routh guilty of capital murder, find him not guilty or find him not guilty by reason of insanity. If convicted, Routh faces life in prison without parole. Prosecutors aren't seeking the death penalty. Even if he's acquitted, Routh could remain in custody. The Texas criminal code stipulates that in cases involving violent crimes where defendants are found not guilty by reason of insanity, the court can initiate civil proceedings to have them committed.