This seems like an irrational act unless you consider this possibility.
If a soldier has experienced the horrors of war with buddies being killed and maimed, and if his personal life is also destroyed by affects of the war on his personality and on his relationship with his spouse and or family..
If he is feeling he has no life to go back to, nothing left.. he could decide the best thing he can do with his life is commit an act so horrific that the citizens of Afghanistan could never forgive the Americans, they would demand our immediate departure..
Then his last act would be to end the war.. the thing he hates most.
I think it is a possibility.
JJ