Mental illness in criminal defenses

To be found culpable or responsible for commission of a crime you have to be able to form the criminal intent require by law. Mental illness is a defense that though often misused is a very real situation for some people.
I believe a great example of mental illness is David Berkowitz “Son of Sam” he claimed to be getting instruction from a dog to kill people. This man was perfectly capable of driving around and making detailed plans to kill others but that does not necessarily make him capable of legal intent.
The legal definition for insanity defense is a legal concept that a person cannot be convicted of a crime if he lacked criminal responsibility by reason of insanity at the time of commission of the crime. The medical definition is unsoundness of mind sufficient in the judgment of a civil court to render a person unfit to maintain a contractual or other legal relationship or to warrant commitment to a mental health facility. In most criminal jurisdictions, a degree of mental malfunctioning considered to be sufficient to relieve the accused of legal responsibility for the act committed.
An eight-state study reported in the Bulletin of the American Academy of Psychiatry in 1991 showed that the insanity defense was used in fewer than 1 percent of the cases of a representative sampling of the state courts. Only about a quarter of those insanity pleas were successful, even though 90 percent of defendants who made the pleas were diagnosed with a mental illness. And so the refinements and the controversy continue. According to JAMA, there are at least 100,000 insane people in this county. If they take to lives of crime, will they get away with murder?