STEPHENVILLE, Texas — Shortly before he was shot to death by a troubled former Marine at a Texas gun range, legendary Navy SEAL sniper Chris Kyle texted a buddy, “This dude is straight-up nuts,” a defense attorney told jurors Wednesday.

A lawyer for Eddie Ray Routh said 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.

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

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

The first day of the highly anticipated trial also included sometimes tearful testimony from Kyle’s widow, who spoke about her husband’s passion for helping veterans, gun safety and opposition to mixing alcohol with gun use.