Remember the name: Deborah Danner.

She was killed by a New York police sergeant on Tuesday in her Bronx apartment. Neighbors had called 911, saying she was acting erratically. A team of officers arrived and, according to the police account, found an agitated Ms. Danner brandishing first a pair of scissors, and then a baseball bat. She took a swing at the sergeant, who shot her twice.

The investigation has just begun, but the case looks bad for the department. Police Commissioner James O’Neill almost immediately placed the sergeant, Hugh Barry, on modified duty, stripped of his badge and gun. Mayor Bill de Blasio said at a news conference that the sergeant had not followed training or protocols for dealing with those with mental illness, and for some reason had neither used his Taser nor waited for specialized officers trained to deal with such situations. Mr. O’Neill said: “We failed.”

Ms. Danner, 66, now joins a tragic group of people whose mental illness leads them into a dangerous, often fatal, collision with the police. She would have been another cipher, another mental-health casualty, her inner struggles known only to her family and friends, but for a remarkable essay she wrote four years ago, “Living With Schizophrenia,” which her lawyer shared with The Times.

To read its six shattering pages is to come face-to-face with the isolation, self-doubt and crushing misery of mental illness. “Even the smartest people/persons in the world could not function in the realm of normalcy with that monkey on their backs,” she wrote. She told of the constant unease of not knowing where her sickness would take her: “What if my medication fails me? I ask myself, will I know if it does? Will the illness overpower its effectiveness? When? Where?”