STEPHENVILLE, Texas (AP) — A former Marine on trial for capital murder in the death of Chris Kyle, the former Navy SEAL depicted in the blockbuster "American Sniper," was numbed by marijuana and whiskey when he fatally shot Kyle and another man, a prosecutor told jurors Wednesday.

Erath County District Attorney Alan Nash said during opening statements at the trial of Eddie Ray Routh that overwhelming evidence points to Routh as the killer of Kyle and his friend Chad Littlefield at a shooting range in February 2013.

Nash described the 27-year-old Routh as "a troubled young man" and said a history of mental illness should not absolve him of being accountable for the deaths.

"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.

The case has drawn intense interest, largely because of Kyle's memoir about being a sniper who served four tours in Iraq. The Oscar-nominated film based on the book has grossed nearly $300 million.

Defense attorney Tim Moore didn't dispute that Routh accompanied the men to the shooting range but said he was insane, spiraling out of control from a history of mental illness and thought he needed to kill the two or they would turn on him.

Routh's psychosis was so transparent that Kyle and Littlefield became alarmed, Moore said. He read to jurors texts that he said the two men exchanged while driving with Routh in Kyle's pickup to the shooting range.

"This dude is straight up nuts," Kyle texted to Littlefield.

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

Moore told jurors that Routh was suffering from severe mental strain that day. "He thought he had to take their lives or he was in danger," he said.

Routh faces life in prison without parole if convicted.