Clint Eastwood has created a very well-made movie. I think the main moral point is muddled, because Eastwood's own moral vision stops at the individual level. It is not wrong, but incomplete.
The issue is laid out in an early scene, when Chris Kyle, the title character, is still a boy. Chris had defended his younger brother from a bully. Later, their father offered this worldview: there are three kids of people in the world - sheep, wolves, and sheepdogs. The father says he is not raising any sheep, nor any wolves. His sons are to be sheepdogs, protectors of the sheep.
As the father lays it out, he is arguing with those who think there are no wolves in the world. This also sounds like a theme in many Eastwood movies, both as director and actor. But this is really a straw man.
It is a false argument in the first instance because there are very few people who argue that there are no bad guys at all. Even I, who often point out that most people are happy and that the world is getting better, do not say that.
It is a false argument for a deeper reason, too. It assumes that people simply are good or bad, weak or strong. There is no nuance as to why some people might take up arms against another, why, for example, people who were grateful that invaders removed their brutal dictator might still resent the invader, especially if the invaders settled in.
Which brings me to the other character missing from Kyle, Sr.'s metaphor: the shepherd. The reasoning person who thinks about the structures for protecting the sheep, the just ways of controlling wolves, and the good of the entire ecology that includes both.
Chris Kyle was a very good sheepdog, as portrayed in "American Sniper". He was not a shepherd.
The world most needs good shepherds.