Insane doesn't mean "does bad things." Insanity means they have no control over their actions due to a disease of the brain. Psychoses and personality disorders are not the same thing and hold wildly disproportionate levels of responsibility.

Look at this interview with Jeffrey Dahmer and this interview with a schizophrenic patient. Dahmer is calm, collected, organized. He knows what he did, why he did it, the consequences of his actions, and experiences reality exactly the same as you or me. The schizophrenic patient is all over the map, spills her coffee while she talks and shows no sign that she knows she's done it, her thoughts are completely unorganized. She has very little or no control over herself.

That's the difference between sanity and insanity. Dahmer isn't insane and that's why he didn't get out of the death penalty. David Berkowitz tried to plead insanity and failed. If you look at Vince Li's background, you see someone who lived the majority of his life normally and healthily and then, out of nowhere, beheaded and ate some guy he didn't know on a bus. That's the definition of a psychotic episode. Sociopaths show a steady progression up to their "big" crimes.

If one day you suddenly get an irresistible urge to murder and eat someone, yeah, you're more than likely insane. If it's discovered that you did it to prove a point or that you started killing small animals and eating them raw then steadily progressed to killing and eating a person, you're going to prison and probably put to death, because that progression shows a level of responsibility insane people don't have.