Who Was Chris Kyle, America's Deadliest Sniper?

Feb. 4, 2013— -- Many Americans are only now hearing about the legendary exploits of former Navy SEAL Chris Kyle, who was killed in Texas Sunday, but Kyle earned a reputation as a hero among his colleagues and a terror to his enemies years ago on distant battlefields.

With more than 150 confirmed kills, Kyle was undisputedly the deadliest sniper in American history before he left the military in 2009. He wreaked such havoc on Iraqi insurgents during the Iraq War that they reportedly nicknamed him the Devil of Ramadi and put a $20,000 bounty on his head.

In his memoir, "American Sniper," published early last year, Kyle wrote about what was to become his most famous shot -- taking out an RPG-wielding insurgent at 2,100 yards. The kill was one of the longest sniper shots in history. At such distances, snipers have to take the rotation of the earth into account when making their calculations.

"Maybe the way I jerked the trigger to the right adjusted for the wind... Maybe gravity shifted and put that bullet right where it had to be," he wrote.

Brandon Webb, who helped instruct Kyle in the SEAL sniper course early in his career, told ABC News Kyle may go down in history "as one of the world's most accomplished military snipers," but he always remained humble.

"He didn't want to publicize the number of kills," Webb said on "Good Morning America." "It was about the lives he saved in Iraq."

In his own memoir called "The Red Circle," Webb writes that Kyle was "a classic example of a Spec[ial] Ops guy."

"A book you definitely do not want to judge by its cover," he said. "A quiet guy, he is unassuming, mild-mannered and soft-spoken… Walk past Chris Kyle on the street and you would not have the faintest sense that you'd just strolled by the deadliest marksman in U.S. military history."

Webb writes that Kyle survived being shot twice, being blown up in six separate Improvised Explosive Device (IED) explosions and receiving "multiple" wounds from rocket-propelled grenades and other explosives.

Scott McEwen, co-author of "American Sniper," said that no matter the number of kills, remembering Kyle for the how many Iraqi insurgents he shot misstates his legacy.

"His legacy is not one of being the most lethal sniper in United States history," McEwen said. "In my opinion, his legacy is one of saving lives in a very difficult situation where Americans were going to be killed if he was not able to do his job."

Kyle's service continued when he was out of the military, after serving four tours in Iraq and earning two Silver Stars.

Webb said Kyle had some difficulty adjusting to civilian life, but rather than slowing down, his struggles led him to help others deal with post-traumatic stress. Kyle was also the co-founder of the non-profit FITCO Cares that gave in-home fitness equipment to wounded veterans.

It may have been during this call to service that Kyle died, serving his fellow veterans. Police said Kyle was gunned down by 25-year-old former Marine Eddie Ray Routh. Travis Cox, director of FITCO, told The Associated Press that he understood Kyle and another friend were helping Routh through PTSD.

READ: Kyle Murder Suspect on Suicide Watch After Being Tasered

Eric Davis, who was Kyle's direct instructor at the SEAL sniper course, said that from his own book and from Kyle's appearance in other military memoirs, Kyle comes off as a consummate professional and an earnest man.

Davis wrote today that the picture Americans are slowly getting of his star student is just right.

"Often times the impression we get from someone's public persona can be very different from who they actually are," Davis wrote on the special operations website SOFREP.com. "[T]he Chris Kyle that you read about in books, watch on interviews, and will be reading about for years to come is not some made-up public persona... [That's] who Chris Kyle really was."

In a September 2012 interview posted on SOFREP.com, Kyle said that 100 percent of the profit he received from his memoir goes to the families of fellow American service members featured in the book who were killed in action.

"The book definitely -- it's my stories and it has their stories in it, but I'm trying to highlight the people who were the true heroes. And I don't consider myself one of those guys," he said.

ABC News' Dean Schabner contributed to this report.