Shooter of American Sniper 'nuts'

Legendary US Navy SEAL sniper Chris Kyle texted a friend, "This dude is straight-up nuts," shortly before he was shot dead by a troubled former Marine at a Texas gun range, a defence attorney told jurors.

A lawyer for Eddie Ray Routh said on Wednesday in opening statements of the man's murder trial that Routh's insanity was so evident that Kyle and his friend Chad Littlefield exchanged texts expressing alarm as the three rode together in February 2013 to a Texas shooting range.

But a prosecutor said even with mental illness, Routh still knew right from wrong.

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

District Attorney Alan Nash described the 27-year-old Routh as "a troubled young man" who on the morning of the killings numbed himself with marijuana and whiskey. But mental illness should not absolve accountability, he said.

"The evidence will show that mental illnesses, even the ones this defendant may or may not have, don't deprive people from being good citizens, to know right from wrong," Nash said.