The screenwriter said he shaped the character based off of his meetings with Kyle and felt like he was far more damaged/affected by his killing than he let on in public. But I doubt Kyle would take a kinder, gentler stance. There'd probably be a lot of "Say it to my face" to the critics. And I have no idea how he could respond to the debunked stories.

Although, before he died, there was supposedly more in the film about his work/life after the war, which would have helped the film.

It's one of my main issues with the decision to use Mustafa. It took the motivation away from him simply wanting to protect his guys (who were from attack all over, not just from one dude), the fact that anyone anywhere could be a threat (which is a huge issue with PTSD), and, in the end, he decided to not go back because he wanted to save his family. The film made it seem like he had some closure but the decision was tougher. (Or, that's how it's been portrayed in what I've read.)

I understand from a plot angle why you'd add him but the film being a character study, I think it was a bad addition.