Judge ShawnDya L. Simpson, 54, agreed to step down after the Commission on Judicial Conduct determined her behavior in court was “erratic and at times intemperate.” It was unusual enough that the State Commission on Judicial Conduct opened an investigation into Judge Simpson, who has served in three boroughs in her 16-year career. That inquiry ended on Monday, with the judge’s announcement that she had agreed to retire in October after acknowledging to the commission that she has an advanced case of Alzheimer’s disease. Judge Simpson’s family declined to comment on the details of her illness. In a separate statement released on Monday, her husband, Jacob Walthour Jr., a hedge fund executive and the chairman of the board of Ebony magazine, said the “early, sudden and hollowing effects” of Judge Simpson’s ailment “serve as a reminder to us all to live life passionately and with purpose every day.”