To understand why the world is terrified of India and Pakistan going nuclear over Kashmir, consider the Chittisinghpora massacre. Everyone knows who did it, but no one can agree on an answer. By BARRY BEARAK Nanak Singh survived the Chittisinghpora massacre; his son and brother did not. Photograph by Raghu Rai/Magnum, for The New York Times. Barry Bearak is co-chief of the New Delhi bureau of The New York Times. Also in This Issue Reagan Writes The former president's recently discovered radio addresses from the 1970's show that he was more his own man than most people recognized.