In a statement to the Los Angeles Times, Kyle's family said part of what he fought to defend was the justice system, which they now must trust to decide the fate of his accused killer.
"Chris and many other young men and women serving in the armed forces have made personal sacrifices to preserve the American way of life. A part of the American way of life includes our justice system, which is not perfect, but is the best in the world," the statement said. "The family has the utmost confidence that a fair and just verdict will result from the upcoming murder trial."
Kyle became one of the country's most lethal snipers during four tours of duty in Iraq. The book he wrote about his experiences, "American Sniper: The Autobiography of the Most Lethal Sniper in U.S. Military History," became an Oscar-nominated movie directed by Clint Eastwood and starring Bradley Cooper.
Routh, now 27, is to appear in court Friday for the second day of jury selection. He has been held at Erath County Jail on two counts of murder and one count of capital murder in lieu of $3-million bail.
Erath County Dist. Atty. Alan Nash has said he will not seek the death penalty. Routh's attorneys have filed motions indicating they plan to pursue an insanity defense, and have questioned whether he can get a fair trial here given the publicity surrounding Kyle. They filed requests for a change of venue that District Judge Jason Cashon, the former district attorney, has so far denied.
Nash and Routh's attorneys declined comment this week based on a gag order issued by the judge two years ago.
Ed Huddleston, a Fort Worth-based attorney for Kyle's widow, said the family released a statement only after checking with prosecutors.
Taya Kyle, 40, and the couple's two children still live in the Dallas bedroom community of Midlothian, where her husband attended high school and where they settled before his death.
A statuesque brunette, she delivered a heartfelt eulogy at her husband's funeral before an audience of thousands.
"God knew it would take the toughest and softest-hearted man on earth to get a hardheaded, cynical, hard-loving woman like me to see what God needed me to see, and he chose you for the job," she said. "He chose well. You taught me innocent, reckless love.... You taught me that I can forgive more deeply than I ever thought I could."
Afterward, Taya Kyle became an inspirational speaker. She wrote an introduction to the book her husband was working on when he was killed, a firearm history text called "American Gun." She also made a surprise appearance at the 2013
"I challenge anyone to tell me there isn't evil in this world. From the days of Cain and Abel, we know all too well there will always be evil, but that evil shouldn't take away our freedoms. In fact, the only way to take away evil is by taking advantage of those freedoms," she said.
Kyle worked with the screenwriter for "American Sniper" and spoke with The Times last month to promote the movie, which explores the stress her husband's repeated deployments placed on their relationship.
At the time, she said of her husband's death: "The way it went down was so wrong, and yet on some level Chris died as he lived: serving."
She dismissed claims that Routh's actions were linked to post traumatic stress that he may have suffered after deploying to Iraq and Haiti as a Marine.
"To try and even find an excuse is disgusting," Kyle said. "I know people with PTSD, and it's very real and very hard. But it doesn't change your core character."
Kyle has been working on a book of her own, "American Wife: A Memoir of Love, Service, Faith, and Renewal," which is due from William Morrow on May 5.
In Thursday's statement, the Kyle family said they were still "anguished by his sudden and incomprehensible death and they continue to miss him each and every day."
"Chris was a wonderful father to his children and husband to his wife, Taya. His violent death is not only a grievous loss to his family but to an entire nation as well. Chris both lived and died trying to help his fellow veterans," the statement said.
So far, 800 jurors have been summoned for Routh's trial - half Thursday and half Friday - and 39 have been eliminated. Attorneys will question the remaining pool of qualified jurors on Monday and Tuesday. Opening statements are expected Wednesday.
The judge has allowed one television camera to film trial proceedings through a courtroom window, with a microphone inside the courtroom, but he has ordered media listening to the audio at the courthouse not to broadcast it.
Erath County, population 40,000, is about 100 miles southwest of Dallas and is mostly rural ranch country, although the county seat where the trial is taking place is a college town.
Huddleston, Taya Kyle's attorney, said he expected her and her relatives to attend the trial. This is an "emotional time," he said, and she is likely to "re-grieve all over again."
Last month, Kyle told The Times she was apprehensive about the trial.
"I have a feeling the trial is going to be a beat-down," she said. "And yet there's no place I'd rather be. Everywhere I can be supporting Chris and standing up for him, I will always be there."
The court has set aside 21 of the 113 seats in the courtroom gallery for relatives of Kyle, Littlefield and Routh. It's unclear how many Kyle supporters will attend. Scores of people, including many veterans, lined a Dallas highway for his funeral procession two years ago, hoisting signs and American flags in scenes featured at the end of "American Sniper."
The family expressed thanks for such support.
"Our family stands strong in God, country and family," the Kyle family statement said, adding, "We are grateful and humbled for the support, prayers and love that we have received during this painful time."