Why should Macbeth kill King Duncan?
Just to take a bit of a different angle concerning Shakespeare's Macbeth, first, no evidence really exists in the play that Macbeth was nurturing evil, corrupt thoughts before he hears the witches' predictions. It seems logical, but it's difficult to definitively declare that. Second, if he was, then Shakespeare doesn't show a fair mind being corrupted subtly and gradually over time, since Macbeth was already corrupted when the play opens. You might be able to argue one or the other, but you can't logically argue both.
That said, again, looking at this issue from a different angle, the witches predict only that Macbeth will be king, not that he should assassinate Duncan to get the crown. Macbeth creates that idea on his own.
Once Duncan names Malcolm as his heir, however, Macbeth has little choice, if he ever wants to be king. Malcolm stays behind the lines during the opening battle with his father while Macbeth does the fighting--Macbeth could hardly expect to outlive Malcolm, and even if he does, there's always Donaldbain.
If Macbeth is obsessively ambitious, he has little choice but to kill the present monarch.
Obviously, he shouldn't. Murder has physical, emotional, and spiritual consequences. The murder of a benevolent King who was also family? Even worse. Macbeth becomes a traitor, a bloodthirsty tyrant, a paranoid sleepless zombie, and a necromancer, not to mention a headless corpse.
Before the play, all was well. Macbeth won victory and favor with the King. But a combination of ambition, carefully planted seeds by the witches and his wife, and Duncan's naming Malcolm Prince of Cumberland cause Macbeth to take on a Machiavellian quest to commit regicide and fratricide (the two worst crimes imaginable for a Thane).
The murder of the King disturbs social and natural order: witches become advisors, horses eat each other, the earth trembles, fair becomes foul, the king murders his best friend, women and children, the royal family sleepwalk and talk to the dead. All in all, a hell on earth and a toppling of the Great Chain of Being.
This is the objective correlative of the play and the theme of the entire play is encapsulated in it. Shakespeare in his "Macbeth" wishes to demonstrate how evil - 'foul' - can subtly and gradually permeate into the mind of a good - 'fair' - person and corrupt it and overwhelm him completely.
The process begins with the witches planting the evil thoughts of unbridled ambition in his mind and his wife manipulating him to commit regicide as a means of usurping the kingdom of Scotland. One must bear in mind that during Shakespeare's time, according to the 'Divine Right of Kings' theory Duncan is actually God's representative on earth and when Macbeth murders Duncan he is actually murdering God himself! This is the reason why Shakespeare makes Macbeth murder Duncan.
Jesus Christ said,
For from within, out of men's hearts, come evil thoughts, sexual immorality, theft, murder, adultery, greed, malice, deceit, lewdness, envy, slander, arrogance and folly (Mark 7:21-22).
Macbeth had all along been nurturing evil ambitious thoughts in his mind and the external stimulus provided by the witches led him swiftly to his ruin.