Routh, who admitted to police that he killed Kyle and the marksman's friend Chad Littlefield, will have his day in court early next month. By then, news of Kyle's life, his "unverifiable" legacy and his tragic death will have already ricocheted around the world, thanks to the box office hit "American Sniper."
The circumstances of Kyle's death aren't discussed in detail in the Oscar-nominated biopic, which mostly focuses on Kyle's career as one of the country's most praised and skilled snipers.
Though the movie is presented as a mini-chronicle of America's modern wars, it omits a chapter in Kyle's real-life story that reflects on war's dark, lingering consequences: His death at the hands of another veteran — one who had been scarred by the effects of war-related mental illness.
Routh's many troubles were the very reason Kyle and Littlefield were in contact with him. Kyle's trip to the Rough Creek gun range that day was likely part of an effort to help Routh deal with his post-traumatic stress disorder.
But at some point that day, Routh, a 27-year-old former Marine, opened fire on the two men, killing them. He took their Ford 350 truck and fled.
"He's all crazy, he's [expletive] psychotic," Routh's panicked sister, Laura Blevins, told the 911 dispatcher that day.
"He was recently diagnosed with PTSD," her husband, Gaines Blevins, added later on the call.
It wasn't the first time Routh's family had called law enforcement pleading for help. He had been in and out of psychiatric wards for years. He had threatened to kill his family and himself — threats that were taken so seriously that a Marine friend removed all the weapons from the house for safekeeping.
"They're all hunting weapons, you know, shotguns and rifles," his mother, Jodi Routh, said in a 911 call to police months before the shooting. "He was threatening to, you know, shoot himself, and I just can't have that. . . . We were trying to get them out of here without him seeing us take them out."
Routh's relatives say he was formally diagnosed with PTSD. And recollections from family and friends suggest that he might have suffered from other mental illnesses as well.
That Kyle would be killed by a veteran who was like so many others he tried to help — troubled by war and struggling to adjust to civilian life — was a tragedy layered upon a tragedy. Kyle had answered the call from Routh's mother to help her son, but Eddie Routh turned on him, shooting him in the back.
The people who knew Kyle best are not interested in the situation's dark irony.
"To try and even find an excuse is disgusting," his widow, Taya Kyle, recently told the Los Angeles Times. "I know people with PTSD, and it's very real and very hard. But it doesn't change your core character."
Yet the troubling connection between war veterans, mental illness and acts of violence is persistent.
A 2014 study found that veterans who had problems with PTSD or alcohol abuse were seven times more likely to engage in acts of "severe violence" than other veterans. And in 2011, a New York Times analysis of some of those cases found dozens of examples of Iraq and Afghanistan veterans accused of murder; some of the veterans even turned weapons on themselves.
There are no current and official nationwide statistics about veterans who enter the criminal justice system.
While there are efforts to create alternative courts and sentencing guidelines for veterans accused of crimes who suffer from PTSD and combat-related mental illnesses, those efforts have been slow to apply to those accused of serious violent crimes.
"Violent crimes are excluded, which to me is a real shame; you should look at it case by case," said Duncan MacVicar, a retired Vietnam veteran who began working with California jurisdictions to establish veteran treatment courts in the state. "Is it really true that someone with PTSD can commit a crime because of that problem? I can guarantee you that it's true."
Routh's version of the story has yet to be fully told. Next month, that might finally happen — with the world bearing witness.
His lawyer, J. Warren St. John, will reportedly pursue an insanity defense. But St. John has already questioned whether he can get a "fair trial" while a blockbuster film about one of the men his client killed plays in theaters nationwide.
St. John's petition to move the trial from Erath County in Texas has already been denied once; it will likely be even more difficult now to find a suitable venue where anyone can claim to not know Chris Kyle's name, or at least his "American Sniper" accomplishments.
It is also unclear whether Routh's diagnosed PTSD and mental illness will ultimately have any bearing on the trial.
And while there is a growing recognition that PTSD can profoundly change or exacerbate the mental state of veterans who come back from war, Chris Deutsch of the Virginia-based organization Justice for Vets says it remains very difficult to balance the need for rehabilitation with a desire for justice in a case where a victim was seriously injured or killed.
"More than ever before, there's a recognition that these issues can have a profound effect on behavior and are profoundly affecting men and women who have no criminal history and no history of violence," Deutsch said of vets with PTSD. "How do you get that person into a situation where they are being connected to treatment and where you're also protecting public safety and ensuring that there's structure and supervision and accountability?"
Either way, both sides will try to disentangle Routh's complicated pre-war past from whatever mental scars he might bear.
Friends who knew him told the New Yorker in a riveting 2013 investigation that as a teen, Routh was a "standard troublemaker" with no respect for teachers:
Kc Bernard, who was a security guard at the school for two of the years that Routh was there, said that Routh was "always ready to fight" and "had a chip on his shoulder."
But his dispatches to family while he was deployed suggest that Routh was also haunted by the death he witnessed in the war zone, including one incident in which he might have killed someone while on patrol.
Routh served four years in the military and was stationed in Iraq from 2007 to 2008 and on a disaster relief mission in Haiti in 2010. He returned to the United States and worked odd jobs, and he was reportedly prescribed eight medications to treat a wide range of symptoms, including depression, mania and nightmares, according to the New Yorker.
His complex psychological profile and troubles with substance abuse that his family coped with upon his return are almost standard for people with PTSD.
"Other than depression, PTSD has more co-morbidities than any other mental disorder," said Edna Foa, a psychologist at the University of Pennsylvania who developed a breakthrough treatment protocol for PTSD. "So it is very common to have PTSD and also depression and also other anxiety disorders, and many other morbidities."
Though not everyone does, some people with PTSD experience flashes of anger as a symptom. When anger on rare occasions turns to violence, it could be because the person is experiencing a flashback, which might to them seem like a potent hallucination, Foa said.
Routh's family doesn't know what changed him, but they know that something eventually did. Their struggle with Routh's mental state continued after he left the Marines until the day he told his sister he "traded his soul for a new truck" that afternoon in 2013.
For Routh's family, the trial will be another painful moment in their long struggle to find help for their loved one.
"I am so sorry for the Kyles and the Littlefields," his father, Raymond Routh, told the Daily Mail this month. "We wrote them letters of apology after it happened but you can't talk to them because there's anger, there's hurt. They want justice.
"How do I explain I want justice for my son too?"
