I can tell you obiously haven't been to war so I am going to try and tread lightly here. War is gruesome and horrific. A place where one instant you are chatting with a friend, and the next you are placing pressure on his chest because shrapnel from an IED ripped through his lung. You obviously have never held the hand of a dying friend who was supposed to return home in only a short months time and witness the birth of his daughter. You see, WAR IS BLACK AND WHITE, you either live or you die. It isn't the real world, and having a real world mentality will get yourself and your men killed. War is pretty simple, you either kill or be killed. Its all pretty self explanatory out there, and if you had been there you would have witnessed it for yourself. As far as what happens to an individuals mindset once in theater, thats another story. It takes a certain type of person to go towards the sound of gunfire. Not a crazy person, or someone with a death wish, but someone who has come to the realization that death is out of their control.
To be blunt, you should find it disturbing, because it is. The whole notion of going somewhere you know you may be killed and will have to kill someone is disturbing. What do think someone in this situations outlook should be? Should you not have a hate and disdain for your enemy? How would you go about controlling the rage that comes with seeing a friend die?
I only say these things to assure you by no means is Chris Kyle a bad person, and yes he was okay with killing people. He was okay with killing people because that was required of him, and it saved other servicemembers lives. I can tell you that he wanted to kill every single enemy combatant that posed a threat to our boys, because I felt the same when I was there. Having a protectors mindset, you don't enjoy the killing as much as knowing that that dirtbag can't take the life of anymore American's.