In Act III, scene ii of Macbeth, Macbeth says "O full of scorpions is my mind... thou knowst that Banquo and his Fleance lives." What does this...

In Macbeth, Macbeth is deeply influenced by a prophesy the witches make when he and Banquo are returning from battle. In Act I, scene iii, Macbeth comments on the day being "foul and fair" (line 38). They have been victorious against the enemy (fair) and so Macbeth is cheerful and yet the weather is so stormy (foul). Of course, this foreshadows later events when the seemingly noble Macbeth will plot to kill his king and anyone who stands in the way of his kingdom - including Banquo. The witches tell Macbeth that not only will he be Thane of Cawdor, but he will be king. This unsettles Macbeth when, due to his valor in battle, Duncan does give him the title, leading Macbeth, with Lady Macbeth's help, to further his "vaulting ambition" (I.vii.27).  

Once Duncan, the king, has been murdered and Macbeth, initially tormented by his own remorse, has realized that, although he is now king, the witches foretold that Banquo's sons would also be kings, Macbeth feels that he must act against Banquo because, as the witches prophecies have been true thus far, then he (Macbeth) is not safe, saying that Banquo has the capacity to spoil everything: "Under him, my Genius is rebuked" (III.i.55). Macbeth is angry that he has possibly murdered "the gracious Duncan" (65) for the benefit of Banquo's heirs. 

Macbeth elicits the help of two murderers and convinces them that they should murder Banquo. He tells them that he would do it himself if he could. He also tells them to be sure to kill Fleance, Banquo's son who must also "embrace the fate of that dark hour" (136). Lady Macbeth notices that Macbeth is preoccupied and tells him not to worry because "What's done is done" (III.ii.12). She has no idea that he is plotting his next murder although he does tell her that the matter is not closed because "We have scotched the snake, not killed it" (13), indicating that his kingdom is in jeopardy. Lady Macbeth encourages Macbeth to "leave this" (35) and not worry; this is when he advises her that his "mind is full of scorpions" (36), knowing that Banquo and Fleance are alive. 

Macbeth is trying to tell Lady Macbeth that he cannot rest or enjoy his kingdom because his mind is unsettled. Just as a scorpion sting would hurt and make a person restless, so too do Macbeth's thoughts. Scorpions inflict much pain and therefore the reference to them is two-fold. They cause Macbeth pain but they also give him the courage to cause others pain in the pursuit of absolute power. So, for Macbeth, the scorpion reference is indicative of how he confuses his quest for power with pain - his own and others.