There is something powerful and transformative that happens when a young guy has to push himself emotionally, physically and spiritually through the intense personal storms that are rights of passage. I know that not all guys experience the following, but many do: subduing and sublimating childhood's tender, anthropomorphic vision of the animal kingdom while shouldering a weapon and killing a large animal for the first time or coming to terms with hurting and being hurt by your childhood playmates once football becomes real, surviving fraternity hazing or military boot camp and numerous other male challenges. In these situations one pushes oneself beyond known boundaries and limits and accepts or rationalizes the consequences.
As a young guy I once shot a dove. Doves are monogamous, and its mate circled above its dying partner, emitting a pitiful call that explains its name. Confused and guilty, I then shot the mate. The death of the second one assuaged my emotional turmoil over killing the first. Killing begets killing, because once the first time is assimilated into the young brain, the next get progressively easier; the trampling of personal and spiritual restraints become rationalized and fall into the background.
So, how must someone tasked with killing human beings with a high-powered rifle, with a scope that allows one to discover the color of eyes, facial tics and religious medals, rationalize and assimilate their actions? How does one survive doing this job? Unless we have done it we will never know; however, although I certainly don't know the answer to this particular question, I do know the answer in the abstract.
The male commenters and reviewers that are casually disparaging Chris Kyle either have never experienced what I mention above or have forgotten. Survival does not just happen in deserts and on oceans and in foreign lands -- it begins on local football fields and parade grounds and fraternity houses and woodlands.
It's possible that Clint Eastwood should never have made this movie, that it is "propaganda", but "WE" sent Chris Kyle out to kill.