In this April 6, 2012, photo, former Navy SEAL and author of the book “American Sniper” Chris Kyle poses in Midlothian, Texas. (Paul Moseley/AP)

He said he killed 160 people, perhaps many more, making him one of the leading U.S. military snipers of all time. In the course of four combat deployments to Iraq, he said insurgents nicknamed him “the devil of Ramadi” and placed a $20,000 bounty on his head.

“After the first kill, the others come easy,” Chris Kyle wrote last year in his best-selling memoir of Iraqi war service with the elite Navy SEALs. “I don’t have to psych myself up, or do something special mentally — I look through the scope, get my target in the cross hairs, and kill my enemy, before he kills one of my people.”

In a career mostly lived by the gun, Mr. Kyle’s death at 38 was nonetheless shocking.

He was killed Feb. 2 in a double slaying at the Rough Creek Lodge and Resort shooting range about 50 miles southwest of Fort Worth. Authorities identified the shooter as Eddie Ray Routh, 25, a military veteran living in Lancaster, Tex. Routh was arraigned on two counts of capital murder, in the deaths of Mr. Kyle and another man at the gun range, Chad Littlefield.

Both men were shot at close range, a Texas Department of Public Safety spokesman said. A motive was unclear.

Mr. Kyle, former Texas ranch hand and bronco buster who called himself the antithesis of the “refined assassin,” joined the SEALs in 1999 and served four combat deployments before retiring in 2009.

The SEALs specialize in surgical strike forces, and Mr. Kyle’s steady nerve, his patience for stalking and his pinpoint marksmanship through his rifle scope earned him two awards of the Silver Star and five awards of the Bronze Star.

Mr. Kyle’s book, “American Sniper: the Autobiography of the Most Lethal Sniper in U.S. Military History,” sold hundreds of thousands of copies. Co-written with Scott McEwen and Jim DeFelice, “American Sniper” rode a crest of interest in behind-the-mystique, preserve-the-mystique SEAL volumes such as “No Easy Day,” Matt Bissonnette’s pseudonymous account of the raid that killed Osama bin Laden.

Mr. Kyle was at the Rough Creek Lodge for a charity event to support his Dallas-based security firm, Craft International. Mr. Kyle also helped start a nonprofit group, the FITCO Cares Foundation, to supply at-home fitness equipment to emotionally and physically wounded veterans.

With his Texas drawl, hulking physique and tightly reserved public manner, Mr. Kyle drew a degree of celebrity in the past year as he appeared on late-night talk shows and in the NBC competition show “Stars Earn Stripes,” which pairs military and law enforcement veterans with actors in drill exercises.

In a Time magazine interview, Mr. Kyle agreed that it was “kind of frowned on” in commando circles to become a public figure. “But I’m not trying to glorify myself,” he said. “I didn’t want to put the number of kills I had in there. I wanted to get it out about the sacrifices military families have to make.”

Christopher Scott Kyle was born in 1974 and grew up on a ranch in Odessa, Tex. As a young man, he hunted deer and pheasant with a bolt-action 30-06 rifle and rode bulls and broncs in rodeos.

“When I grew up, I only had two dreams,” he told the Dallas Morning News last year. “One was to be a cowboy and another was to be in the military. I grew up extremely patriotic and riding horses.”

After studying ranch and range management at Tarleton State University in Stephenville, Tex., he returned to ranching before enlisting in the Navy with the ambition of joining the SEALs.

He recalled his first kill in Iraq, of a woman walking with a child. Through his scope, he watched her remove a grenade from under her clothes, just as a group of Marines was approaching. It was a disquieting experience that made him hesitate at first from firing.

“She had turned herself into a suicide bomber,” Mr. Kyle later told the Fort Worth Star-Telegram. “Her intention was to kill herself and blow up the Marines. . . . Either way she was going to die.”

His sniper shots ranged mostly from 200 to 1,200 yards. Remarkably, while on assignment near Sadr City in Baghdad in 2008, he managed to kill an insurgent from about 2,100 yards away. The fighter was about to launch a rocket-propelled grenade at an Army convoy and, besides distance, Mr. Kyle had to consider factors such as wind and vibration from the shot.

“God blew that bullet and hit him,” Mr. Kyle told the New York Post.

Amid the bravado, “American Sniper” explored the impact of military life on Mr. Kyle’s marriage.

On an assignment in Baghdad, Mr. Kyle wrote in the book, he called his wife on a satellite phone only to be interrupted by burst of gunfire and then a rocket-propelled grenade. The phone stayed on for much of the battle, then went dead.

It was days before Mr. Kyle could call his wife and reassure her he had come through. He wrote that he left the military to save his marriage. Besides his wife, Taya, survivors include their two children. A complete list of survivors could not be determined.

The Pentagon does not typically confirm sniper kills publicly, but Mr. Kyle wrote that the Navy credited him with 160 kills as a sniper.

That would make him a vastly more accomplished sniper than the previous known U.S. record-holder, Adelbert F. Waldron III, who was credited with 109 kills in Vietnam. Finnish sniper Simo Hayha killed more than 500 Soviets during World War II, by most estimates a world record in known combat operations.

“American Sniper” provided rare insight into a sniper’s telescope-lens view of the world, where there exists by the nature of the work only friend and foe.

“For the most part, the public is very soft. You live in a dreamworld,” he told Time magazine last year. “You have no idea what goes on on the other side of the world, the harsh realities that these people are doing to themselves and then our guys. And there are certain things that need to be done to take care of them.”