I think one of the main differences regarding the law cases about mental illness and insanity is that mental ilness it is not a legal defense like insanity is.
If a person is found to be insane, the person is not guilty of the offense. If a person is mentally ill, then the person is guity of the offense but will almost certainly receive a lesser punishment at sentencing.
What I found on Internet is that, criminal insanity and mental illness are two different things. An individual who is criminally insane might show a pattern that is violent and against the law. Usually having mental illness doesn't mean a sufferer is violent and dangerous,most of the time they are having battles with themselves.