I posted this before the film was released but my good friend wrote this and another friend produced it. Both are riding a wave right now that is pretty freaking unbelievable, Jason may very well win the Oscar for adapted screenplay. Whatever you think of the movie, it's a great adapted script.
Five years ago, I was doing some work on his house and every time I walked past the room he used as an office, he was sitting there with his head in his hands, staring at his laptop. Not typing, just staring. Sometimes an hour or two would pass and he hadn't moved. Writing is just grueling, tortuous work, IMO. I lose my mind when I try it, I need to be moving and using hand tools.
He has a wife, 3 small children and a Los Angeles mortgage plus a leased Chevy Tahoe. The most he'd ever done before this was some crap movie with Ashton Kutcher. He optioned the film rights for Chris's book and was working on it with him when Chris was murdered. They were texting each other up to almost the minute it happened. When that occurred, he was left to work with Taya, (Chris's widow), and decided to shift the focus of the story somewhat to their relationship and the cost soldiers and sailors pay in personal terms when deployed to far away locales in harm's way. The result was a pretty amazing script.
There are a bunch of other interesting stories about Jason hanging out with Chris and his SEAL team buddies, who of course did not trust or accept an outsider, much less a Hollywood writer. At one point, he challenged one of them to wrestle on a cement pool deck and won, that went a long ways towards warming things.

How were they to know that he was an all-state wrestler in HS? And CA is a big state.
Anyhow, I respect that people have divergent opinions about Chris Kyle and the film. He was a complicated person with some questionable qualities but he believed in what he was doing all the way. I think that he was someone whose views may have evolved on certain things had he lived, but we'll never know now. The film is doing big business, that's for sure.