A film glorifying him seems to be in bad taste (I didn't see it), but at the end of the day I do think there's some merit in the argument that soldiers set aside morality and ethics on the battlefield and just do what leaders tell them to do. Chris Kyle didn't start the war; he didn't invent the position of sniper, he just did what he was called on to do, and he was very effective at it.
Hasn't "I was only doing what I was told" been considered an illegitimate excuse for one's actions since Nuremberg?
Not really. If you had signed up to Iraq and killed someone there, an "enemy combatant" during a military offensive, you could still have been welcomed home as a hero, even though you committed an intentional homicide against someone who might have only been trying to defend his home (e.g., not justified in a civilian environment). The only difference with Chris Kyle is that he is that person x250. We send people over to kill, and we call them heroes for killing. Kyle did it in spades, but functionally he only did the same thing many, many other veterans have done.
Edit: Extrapolate that, and you get Vosem's viewpoint. I'm not saying I agree with it, but there is a logic to it.