What Elizabethan playwrights learned from the Greek classics was not theories of insanity, but dramatic practice - that is, madness is a dandy theatrical element. It focuses the audience's attention and increases suspense, since you never know what a mad person may get up to next; and Shakespeare himself makes use of it in many forms. In King Lear, there's a scene in which one man pretending to be mad, another who has really gone mad, and a third who has probably always been a little addled, are brought together for purposes of comparison, irony, pathos, and tour de force acting. In Hamlet, there are two variations - Hamlet himself, who assumes madness, and Ophelia, who really does go winsomely bonkers. In MacBeth, it's Lady MacBeth who snaps.