"You're a little more aggressive when you're at work and when you come home, you relax and try to be the different person," he said.
Kyle helped established the FITCO Cares foundation, a charity that helps U.S. war vets "who have survived combat but are still fighting to survive post-traumatic stress disorder," the group's website said.
Life back home was a challenge for Kyle, who acknowledged that he turned to alcohol for comfort at one point. "After I was discharged from the military, it was difficult trying to become a civilian," he told a lawyer during a deposition for a lawsuit last November.
"You're in a combat zone one day," he said. "You come home, and then you have to readjust, and it takes a few days. We just sit in the house, hang with the family and then things get better. But it's simple things of trash blowing across the road, reminded of an (improvised explosive device), you might want to swerve. So that's why you just stay at home."
Former Minnesota Gov. Jesse Ventura filed a lawsuit last year accusing Kyle of defaming him in the book by exaggerating his description of a fight between the two at the wake for SEAL Mikey Monsoor, who was killed in Iraq in 2006.
The book quoted Ventura, also a Navy veteran, as saying he "hates America" and telling Kyle, who was mourning the death of a SEAL teammate, "You deserve to lose a few." Kyle described punching Ventura out at the Coronado, California, bar.
In the suit, Ventura denied making the statements in the book, contending that "the entire story about a confrontation with and physical assault and battery of Governor Ventura was false and defamatory."
At his deposition last November, Kyle continued to insist his book accurately described his clash with Ventura.
"He was complaining about the war, that we shouldn't be there," Kyle testified. "Complaining about Bush, that, you know, Bush was a war criminal. How we were killing innocent men and women and children overseas."
Kyle acknowledged in his deposition saying that he hated Ventura "with a passion."
His relationship with another politician was warmer. His company did some security work for former Alaska Gov. Sarah Palin, whom he met while on the "Stars Earn Stripes" reality TV show with her husband, Todd.
"Chris was a wonderful man, a good friend, and a true American hero who loved our country and served honorably. He was loved and admired by so many, and he will never be forgotten," the Palins said in a statement Sunday.
Much of Kyle's time since leaving the military was spent building Craft International, which offers military training for law enforcement and provides security services.
His company also organizes "civilian and corporate shoots" at gun ranges, he said. "It is only fun day shoots, self-defense, or weapons familiarization," he said.
In a Guns.com interview at a gun dealers convention in Las Vegas last month, Kyle was asked about President Barack Obama's gun control proposals, which he said he believed to be aimed at "trying to ban everything."
Banning 30-round magazines for assault rifles would be "opening the door to start taking more of our rights," he said.