The law in all 49 other states is different. In every state except Massachusetts, the defense must prove insanity beyond a reasonable doubt
In Massachusetts, the state must prove sanity beyond a reasonable doubt.
It’s logically incoherent. In CA for example, the defense has the burden of proof on all affirmative defenses, but the standard of proof is the preponderance of the evidence so a bona fide defense is relatively easy to prove. There’s no logical reason to treat insanity differently