when ever I saw smiling face of a solder coming from "battle" I felt sorry for him...I knew he is not normal (in his mind) any more...

You didn't "know" it - you assumed it, and it seems a bit harsh. Cf.:

http://www.files.trailblazersww2.org/division_photos_page1.htm

 These US soldiers are celebrating the fall of Saarbrucken in 1945. Smiling is a pretty natural reaction, it's probably  relief at still being alive - and maybe to not having to kill.

Laughing is also a way of coping in war, you might be surprised by the context of this photo:

AFGHANISTAN - MARCH 15: British soldiers celebrate the lives of fallen comrades by sharing personal stories following a memorial service March 15, 2007 in Kajaki, Afghanistan. Helmand province . British troops gathered on a mountaintop observation post to honor three British soldiers killed by Taliban attacks in Afghanistan. L/Bdr Russ Clark, L/Bdr Liam McLaughlin and Sgt. Major Mick Smith died in the first week of March, a week that saw a total of four British soldiers killed in combat operations in Helmand. (Photo by John Moore/Getty Images)

http://www.daylife.com/photo/0gvo4QS5mFgPy