The problem though is that almost all societies, before us, considered stupidity to not be an excuse for any crime, for similar reasons to "Ignorance of the law is no excuse." This was because, as another principle I'm forgetting the name of points out, stupidity is indistinguishable from competently-executed malice. If competently-executed malice is always explained away as stupidity, there's no such thing as malice. (The only obvious exception, of course, being manifest insanity.)