I would respectfully disagree that salvation lies in the direction of lowering ourselves to the level of evil of the Muslim... and I would point out that even Kyle did not. In their most intense form they will kill anyone they don’t like. We, in contradistinction, still insist on a Western idea of responsibility and kill only key figures. And that rests upon a sane Jehovah, not an insane Allah.
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?
Letting our evil enemy win is, in some ways, tantamount to adopting his evil if only because we permit it to continue to exact its price on our own soldiers.