Investigators said it appears a man was protecting himself from his stepson when he fatally shot the 41-year-old Wednesday night. Police said David Black, 41, suffered from schizophrenia and hadn’t been taking medicine. When police arrived, the older man told investigators he had shot his stepson in the chest during an argument after Black approached him with a knife and refused to drop it. The shooting happened in the 5100 block of Pin Oak Drive on the northwest side. Black was pronounced dead at Wishard Memorial Hospital.