When a man opened fire at a hospital outside Philadelphia, fatally shooting his caseworker and wounding his psychiatrist, the doctor saved his own life and probably the lives of others by pulling out a gun and shooting the patient. If Dr. Lee Silverman's decision to arm himself at the office was unusual, the violence that erupted at Mercy Fitzgerald Hospital served as yet another illustration of the hazards mental health professionals face on the job — and, experts say, the need for hospitals to do more to protect them.