Yes, I would agree with you, and if you lose your soul in the process of fighting an enemy who have lost their soul - arguably you are rendering your own life meaningless. That said, I certainly don't think that's what is happening with our military while fighting this enemy who have no problem with strapping bombs on woman and children, beheading people, and flying planes into buildings filled with civilians.
I think we also need to realize what it's truly like when exposed to the things people like Kyle were exposed to. It's easy for someone like Maher to sit in a nice cozy studio with a pandering audience and ‘speak his mind’, passing judgment on others who put their lives on the line fighting to protect his American rights to speak his mind. Killing is killing, and during WWII we bombed and killed lots of civilians. War is horrible, which is why deterrent strategies (e.g. having a strong military, having a strong foreign policy and ties to strong like-minded allies, having great intelligence and counterintelligence, and not a policy of appeasement) are essential - to avoid the tragedy of war.
Being true to ones religious convictions in all of this is very, very difficult. Thou shall not kill is pretty clear, but what do you do when someone wants to kill you, and your family, and destroy your way of life?
“Does loving your enemy mean not punishing him?.... It is, therefore, in my opinion, perfectly right for a Christian judge to sentence a man to death or a Christian soldier to kill an enemy. I always have thought so, ever since I became a Christian, and long before the war, and I still think so now that we are at peace.It is no good quoting “Thou shalt not kill.” There are two Greek words: the ordinary word to kill and the word to murder. And when Christ quotes that commandment He uses the murder one in all three accounts, Matthew, Mark, and Luke. And I am told there is the same distinction in Hebrew. All killing is not murder any more than all sexual intercourse is adultery. ...the Christian in arms for the defense of a good cause - is one of the great Christian ideas. War is a dreadful thing, and I can respect an honest pacifist, though I think he is entirely mistaken.” (Mere Christianity, p. 106, 107)
“Thou shall not kill is pretty clear, but...”
It’s not killing that is against the LORD, it is murder. Murder is a unique definition of killing. How would the same God that commanded His people not to kill only a few short years earlier command His people to kill the occupants of the land they were to inhabit?