I’m curious why you jumped to the legal argument. If someone is unaware of the consequences of their actions, it becomes difficult to hold them legally or morally accountable. This is also the basis for an insanity defence.
While having beliefs that don’t match the real world isn’t inherently mental illness - it definitely is beyond a certain point. The clinical definition can be found in DSM5 297.1 (F22).
It’s the difference between “I believe Jesus is my personal lord and saviour” (delusional but understandable) and “I am Jesus… No I can’t swim, but watch me step off this boat and walk on water.” (delusional to a clinically relevant, harmful degree).