When Matthew Sepi returned from Iraq a few months ago, he spoke to his family reluctantly of gunbattles and the "weird noises" children make when they die. He never told relatives whether he killed anyone during combat but said he recently had been diagnosed with post-traumatic stress disorder and had been placed on a waiting list for treatment. To help shield his psyche from images of bodies, family members said, the 20-year-old soldier had adopted a simple technique: Just don't think about it. But early Sunday morning, Army Spc. Sepi found himself thinking about killing in front of homicide detectives....