Definition of insanity under the model penal code. "The Model Penal Code, published by the American Law Institute, provides a standard for legal insanity that serves as a compromise between the strict M'Naghten Rule, the lenient Durham ruling, and the irresistible impulse test. Under the MPC standard, which represents the modern trend, a defendant is not responsible for criminal conduct "if at the time of such conduct as a result of mental disease or defect he lacks substantial capacity either to appreciate the criminality of his conduct or to conform his conduct to the requirements of the law." The test thus takes into account both the cognitive and volitional capacity of insanity." So this guy would have to show believable evidence that at the time he shot the guy, he was suffering from a mental disease or defence such that he didn't have the capacity to understand that what he was doing was illegal. Most people realize that shooting someone is not legal in all but a few circumstances.