Kyle, a former ranch had turned SEAL credited with the most confirmed kills of any U.S. military sniper, has been lionized in his home state of Texas. He became entrenched in U.S. popular culture in large part due to his best-selling book and the Academy Award nominated movie directed by Clint Eastwood and starring Bradley Cooper.
After leaving military service, Kyle, who battled his own mental demons, helped counsel troubled veterans with trips for shooting and talks. He had driven Routh to the range with Kyle's neighbor, Littlefield.
"You took the lives of two heroes, men that tried to be a friend to you. You became an American disgrace," Littlefield's brother-in-law, Jerry Richardson, said of Routh at a county court in the rural Texas city of Stephenville after the sentence was handed down.
The jury deliberated for a little more than two hours before reaching a verdict. Prosecutors chose not to go for the death penalty and had been seeking a life sentence without parole.
Defense lawyers argued that Routh was a paranoid schizophrenic and should be declared innocent by reason of insanity.
Even though Kyle's autobiography has been criticized for language seen as demeaning Iraqis and has been on the losing end of a defamation suit brought by former Minnesota Governor Jesse Ventura, Kyle is revered in Stephenville, near where he grew up and went to college.
At the Texas State Cemetery in Austin, where Kyle is buried along with other major figures in the state's history, his grave has been the most visited after his body was brought to the site in a flag-waving procession that spanned about 200 miles of Texas highway in 2013.
In closing arguments, prosecutor Jane Starnes said Routh acted coldly and deliberately when he gunned down the two and then plotted his escape.
"That is not insanity. That is just cold, calculated capital murder," Starnes said.