Health experts say they are hardly astonished that the suspect in the worst mass murder ever at a U.S. military base is an Army psychiatrist — the very person who is supposed to be helping soldiers deal with the traumatic stress of war. Maj. Nidal Malik Hasan, suspected of gunning down 13 and wounding 30 at the Fort Hood Army Post in Texas, treated soldiers at the Darnall Army Medical Center there after being transferred in July from the Walter Reed Army Medical Center, where he had worked for six years. Dr. Robin Kerner, an attending psychologist who specializes in...