SAGAMIHARA, Japan — A young Japanese man went on a stabbing rampage Tuesday at a facility for the mentally disabled where he had been fired, officials said, killing 19 people months after he gave a letter to Parliament outlining the bloody plan and saying all disabled people should be put to death. When he was done, Kanagawa prefectural authorities said, 26-year-old Satoshi Uematsu had left dead or injured nearly a third of the almost 150 patients at the facility in a matter of 40 minutes in the early Tuesday attack.