"Blood hath been shed ere now, i' th' olden time, ere humane statute purged the gentle weal; ay, and since too, murders have been performed too terrible for the ear...This is more strange than such a murder is" Macbeth begins to go insane as he sees Banquo's ghost at a dinner whereas no one else can see it. At that point Macbeth says that it's not his fault that Banquo has been killed and it is the beginning of the insanity that Macbeth suffers as a result of killing Banquo and the king.
