In Macbeth, how is killing a king sacrilegious, and how does it highlight the parallel between Macbeth's crime and Adam's sin?
In earlier centuries, it was believed that kings ruled by "divine right." In other words, they ruled because it was God's will that they occupied the throne. When King Duncan uses the first person plural pronoun "we," as he always does, he is referring to God and to himself as God's representative as monarch. Killing a king, then, becomes not just a crime against the political state but a sin against God.
In the Book of Genesis, the story is told that Adam and Eve defy God's will and eat forbidden fruit from the Tree of Knowledge. Their fall from grace introduces sin into the world, thereafter known as "original sin," and determines that man's basic nature thereafter will be evil. The evil in Macbeth's nature is his ambition. He murders King Duncan, coldly and deliberately, to gain power. Macbeth fulfills his own will, as did Adam and Eve, by choosing to ignore God's will. Macbeth knows full well the spiritual consequences of his decision; he acknowledges that he has given his "eternal jewel," his soul, to "the common enemy of man," the Devil himself.