STEPHENVILLE, Texas » While defense attorneys mount an insanity defense for the former Marine on trial in the shooting deaths of "American Sniper" author Chris Kyle and his friend Chad Littlefield, prosecutors have described Eddie Ray Routh as a troubled drug user who knew right from wrong. Criminal law experts say the case hinges on whether the defense can prove Routh did not know that the killings constituted a crime.

Routh’s attorneys have said the 27-year-old, who was diagnosed with post-traumatic stress disorder and was taking anti-psychotic medication, was insane when Kyle and Littlefield took him to a shooting range to provide support and camaraderie. Routh, his lawyers say, believed the men planned to kill him.

Because Routh was at a shooting range and both Kyle and Littlefield were armed, "it’s an easier case than others for the proposition that he believed they were about to kill him," said George Dix, a criminal law professor at the University of Texas, Austin.

If convicted, Routh faces life in prison without parole. Even if he’s acquitted, Routh would likely 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.

The case has drawn intense interest, largely because of the Oscar-nominated film based on Kyle’s memoir that details his four tours in Iraq.

Prosecutors did not seek the death penalty — "presumably because they thought it wouldn’t stick," said Andrea Lyon, a defense attorney and dean of the law school at Valparaiso University.

Though prosecutors appear to have conceded that Routh was mentally ill, they will argue that he was competent enough to make choices, Lyon said.

"The question is: Is he legally responsible?" she said.

Erath County District Attorney Alan Nash said during the trial’s opening statements that the evidence would show that mental illnesses "don’t deprive people of the ability to be good citizens, to know right from wrong, to obey the law."

A Texas Ranger testified Friday that authorities found marijuana, a near-empty bottle of whiskey and anti-psychotic medication while searching Routh’s small, wood-framed home in Lancaster after the shooting. Routh’s uncle testified that he’d smoked marijuana with Routh the morning of the shootings.

Prosecutors may try to show that Routh deliberately put himself in a more violent state, said Park Dietz, a forensic psychiatrist who examined acquitted killer Andrea Yates.

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

Former Erath County sheriff’s deputy Gene Cole testified Friday that after Routh was jailed, he heard him say: "I shot them because they wouldn’t talk to me." He said Routh said he had been riding in the back seat on the way to the shooting range. Cole, who is now a police officer elsewhere, said Routh also said, "I feel bad about it, but they wouldn’t talk to me. I’m sure they’ve forgiven me."

Routh’s mother had asked Kyle, a former Navy SEAL whose wartime exploits were depicted in his 2012 memoir, to help her son overcome troubles that had at least twice led him to be admitted to a psychiatric hospital. Routh had been a small arms technician who served in Iraq and was deployed to earthquake-ravaged Haiti before leaving the Marines in 2010.

Prosecutors presented a video Thursday showing officers speaking with Routh as he sat in the pickup that authorities said he took from Kyle after fleeing the shooting range where Kyle and Littlefield were killed. Routh refused to leave the vehicle and eventually sped off with police in pursuit.

The video shows that a police vehicle rammed the pickup. Eventually the car became disabled along the side of the road.

The video showed officers trying to talk Routh into surrendering as he makes comments such as, "Anarchy has been killing the world," ”I don’t know if I’m going insane," and, "Is this about hell walking on earth right now?"

Lancaster police Lt. Michael Smith said Routh "told us he’d taken a couple of souls and he had more souls to take."

Authorities say Routh had earlier driven to his sister’s house, admitted to the killings and told his sister, "People were sucking his soul."

After presenting the video Thursday, prosecutor Jane Starnes said: "I think we can agree he makes some kind of odd statements."

Defense attorneys have noted that even Kyle had described Routh as "straight-up nuts" in a text message to Littlefield shortly before they were killed.

Stengle reported from Stephenville; Schmall reported from Fort Worth, Texas.