It’s harder than it used to be I think. Before psychotropic drugs actually started working, insanity was kind of permanent and incurable. If someone was deemed not guilty by reason of insanity, they were likely going to spend the rest of their life in a State mental institution. And trust me—there were many people who would rather be in prison than a mental institution. As late as the 1960’s those places were bad, bad places to be. It was legal for the medical staff to do all kinds of crazy stuff to you, like stick an ice pick up your nose and bash out part of your frontal cortex. (aka frontal lobotomy)
Things are getting blurry now because we really have drugs out there that can make people sane. You could honestly find someone who’s batshit crazy and killing people because they think they are demons from another dimension, give them a pill and watch them slip back into sanity in a manner of weeks or even days. Heck, I’ve heard with some injectable drugs like Haldol, they can go back to reasonably sane within hours. We didn’t have to think about the possibility of someone insane becoming a sound and reasonable person again because it just didn’t happen. These cures for psychosis are relatively new in the eyes of the law.
Also lawyers are now slicker than they used to be. Honestly, you could claim that ANYONE who commits a murder is insane. I mean, what kind of sane person would murder someone else? So every murderer out there has a loophole now they can jump through if they have a tricky enough lawyer. That’s why in confessions, they make a big deal of asking the person if they knew what they did was wrong but they did it anyway. It leaves a lot less wiggle room for lawyers to make an insanity plea.
I wish I had a good answer, but right now we’re just in a tough time with the law and psychosis (or insanity.., whatever you want to call it). I think things might change in the future, once its more recognized by the law that insanity is now almost totally curable.