Maryland is one of the states that allows a defendant in a criminal trial to plead not guilty by reason of insanity, but states differ in how they define insanity from a legal standpoint. Insanity as a legal defense does not mean exactly the same thing as having a mental illness.
In fact, a person may have one or more diagnosed mental illnesses and be unable to successfully use the insanity defense. This was the case with a man who shot two other men on a gun range in Texas in 2013. The man had been diagnosed with PTSD and schizophrenia. However, because he admitted to a Texas ranger that he knew what he did was wrong, the man was not considered insane by legal standards. States use different rules for assessing whether a defendant is insane or not. In Maryland, individuals with a diagnosed mental illness who cannot control their impulses may be able to use the insanity defense even if they know their actions are wrong.
In the Colorado trial of James Holmes, who opened fire in a movie theater in 2012, the defendant and his attorneys are also attempting to use the insanity defense. If the defense is successful, Holmes will go to a mental health facility instead of facing death row.
When charged with a violent crime, a person may wish to use the insanity defense in some cases. For example, if the individual has schizophrenia and did not know what they were doing, an insanity defense may be successful. This could allow the defendant to be treated rather than going to prison. Those who are in such a situation might want to explore this and other possible strategies with a criminal defense attorney.