"There are three types of people in this world: sheep, wolves and sheep dogs."

As spoken by Chris Kyle's father in the Clint Eastwood directed film "American Sniper" (2014), said to be "loosely based" on the real live story of Christopher Scott Kyle 1974-2013. The real Kyle was a highly decorated Navy SEAL sniper who served four tours in Iraq with over 150 confirmed kills. A hero in any sense of the word. 

I don't know anything about Kyle except the portrayal in the film and the Wikipedia page about him. And I mean no disrespect, but based on the narrative in the film Kyle is just as messed up, just as much a butcher as the Iraqi portrayed in the film who used a drill and amputations to terrorize. Kyle's weapon is a bullet, from up to a mile away.

Let's revisit that quote from early the the film:

Wayne Kyle: [to his sons] There are three types of people in this world: sheep, wolves, and sheepdogs. Some people prefer to believe that evil doesn't exist in the world, and if it ever darkened their doorstep, they wouldn't know how to protect themselves. Those are the sheep.

Wayne Kyle: Then you've got predators who use violence to prey on the weak. They're the wolves.

Wayne Kyle: And then there are those blessed with the gift of aggression, an overpowering need to protect the flock. These men are the rare breed who live to confront the wolf. They are the sheepdog.

https://www.imdb.com/title/tt2179136/quotes?item=qt2359001

Chris Kyle has been raised to be an aggressor. He's acclimated to killing and guns as a child, he finds ordinary life unsatisfying and joins the SEALS because he's "looking to be of service" and "likes to fight".