There have been a series of confusing and incoherent explanations for Routh's motives the afternoon of February 2, 2013.
Kyle secretly texted Littlefield, sitting next to him in a pickup, to say Routh was "straight up nuts." Kyle's wife, Taya, "could tell something was up" that afternoon when she spoke to her husband. Kyle sounded "irritated," she has testified, then he never responded to a text asking if he was OK.
Yet Routh didn't reveal a clear reason in the fatal shootings. Kyle was shot four times in the back and once in the face. Littlefield was shot five times in the back.
Instead, the jury heard a series of bizarre explanations for why Routh allegedly turned the guns on Kyle and Littlefield.
Insanity or personality disorder?
Routh's attorneys said the former Marine suffers from psychosis, paranoia and schizophrenia. Prosecutors argued that Routh is a "troubled man" who struggles with a "personality disorder," not insanity.
The arguments were in direct contrast to one another. While Dr. Randall Price, a forensic psychologist who testified as the prosecution's medical expert, said Routh's heavy marijuana use caused a "substance-induced psychotic disorder," Dr. Mitchell Dunn said his opinion was that Routh's "psychosis was not substance-induced."
"It is my opinion that he was suffering from a severe mental disease or defect that caused him not to know his conduct was wrong," Dunn testified.
One piece of evidence -- video from the police car after Routh's arrest -- shows the accused shooter complaining of mental problems and confusion.
"I've been so paranoid-schizophrenic all day, I don't know what to even think of the world right now," he told the officer. "I don't know if I'm insane. Or sane. I don't know what's even sane in the world right now."
'They wouldn't talk to me'
Gene Cole worked as an officer in the Erath County Jail where Routh has been held since his arrest in the killings. Cole testified he heard Routh talk about the shootings four months after the tragedy.
"I heard Mr. Routh say, 'I shot them because they wouldn't talk to me. I was just riding in the back seat of the truck and nobody would talk to me. They were just taking me to the range so I shot them. I feel bad about it, but they wouldn't talk to me. I'm sure they've forgiven me,' " Cole said.
In May 2013, Routh did a phone interview from jail with New Yorker magazine writer Nicholas Schmidle. Portions of the recorded conversation were played for the jury.
Routh described being annoyed with Kyle and Littlefield as they made the 90-minute drive from his home to the countryside gun range.
Routh didn't trust the Navy SEAL and his friend after their first meeting, he told Schmidle.
"That's how I felt that day," Routh said on the recording. "It was the smell in the air that morning. It smelled like s**t."
Routh continued, "It smelled like sweet cologne. I guess it was love and hate, you know. I was smelling love and hate. They were giving me some love and hate."
Later in the phone call, Routh complained that Kyle and Littlefield stopped to feed him at a fast-food restaurant on the way to the range. It was "strange" because he wasn't hungry, and he felt the two men were trying to "force feed" him, Routh said to Schmidle.
'This isn't a spectator sport'
Routh's attorneys have suggested it was the arsenal of firearms lying next to Routh that triggered his psychosis and paranoia on the car ride.
"At the time of this tragedy, Eddie Routh was insane," defense attorney Tim Moore said. "Not only was he suffering from a severe mental disease or defect, not only did he not know his conduct was wrong, he thought he had to take their lives because he was in danger."
Routh said that when the men started shooting pistols at the gun range, he saw it as a showdown.
"So we're shooting pistols here, huh?" Routh recalls telling Kyle and Littlefield during his conversation with Schmidle. "Again, that's pretty much saying, 'Duel mother****er.' "
When Schmidle asked what triggered him picking up a firearm and shooting Kyle and Littlefield, Routh replied that he was angry that Littlefield would not participate in the shooting.
"I asked him a couple of times, 'Hey, are you gonna shoot?' " Routh said. "This isn't a spectator sport. It's a shooting sport. You shoot. And that's what got all, you know, wired up."
After that exchange, Routh said, "I took care of business and then I got in the truck and left."
After deliberating less than three hours, a Texas jury found Routh guilty of capital murder in the case. Judge Jason Cashon immediately sentenced him to life in prison without parole.