What does the M'Naghten Rule mean in regards to the current laws of offenders with mental illness?

M'Naghten fired a pistol at the back of Peel's secretary, Edward Drummond, who died five days later. The M'Naghten ruleis a test for criminal insanity. It is a test where any variant of the 1840s jury instruction in a criminal case when there is a defense of insanity is applied to determine whether a person accused of a crime was sane at the time of its commission and, therefore, criminally responsible for the wrongdoing.