"The first time, you're not even sure you can do it," he said in the interview. "But I'm not over there looking at these people as people. I'm not wondering if he has a family. I'm just trying to keep my guys safe. Every time I kill someone, he can't plant an (improvised explosive device). You don't think twice about it."
At one point, Kyle wrote, he shot a woman who was carrying a grenade while with her toddler. But he did not kill a child in Baghdad's Sadr City area who had a rocket-propelled grenade launcher. "According to the rules of engagement at the time, you could kill anyone with an RPG on sight. That day I just couldn't kill the kid. He'll probably grow up and fight us, but I just didn't want to do it.
He said the American public lives "in a dream world. 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 to our guys. And there are certain things that need to be done to take care of them."
After leaving the military, he founded Craft International, a military training company. Kyle also spoke up on current events, including accusing President Barack Obama of being "against the Second Amendment" because of his gun control initiatives, according to a video interview with guns.com.
The married father of two children established the nonprofit Fitco Cares Foundation to help veterans battling PTSD get access to exercise equipment.
Littlefield, who leaves behind a wife and children, was a friend and another veteran who worked to help people with PTSD, said Fitco Director Travis Cox.
In a statement, the foundation described Kyle as an "American hero" and pledged to carry on his mission.
"What began as a plea for help from Chris looking for in-home fitness equipment for his brothers- and sisters-in-arms" struggling with PTSD turned into an organization that will continue after his death," Cox said in a statement.
"Chris died doing what he filled his heart with passion -- serving soldiers struggling with the fight to overcome PTSD. His service, life and premature death will never be in vain. May God watch over his family and all those who considered Chris a true friend."
His friend, Jason Kos, offered similarly glowing sentiments, telling CNN's "Early Start Weekend" that Kyle was "a man of incredible character."
"He led by example," Kos said. "He always stopped to take time to talk to whoever was around him. Just incredibly humble, very funny as well."