We have scotch’d the snake, not kill’d it;
She’ll close and be herself, while our poor malice
Remains in danger of her former tooth.
– William Shakespeare
Macbeth, Act 3, Scene 2. The snake is a metaphor for the danger the Macbeths find themselves in after Duncan’s murder. Macbeth doesn’t feel safe on the throne and sees Banquo as a threat to his position as long as he is alive. This passage is ironic, since it was Macbeth who created the snake in the first place with his uncontrollable desire for power that pushed him to regicide.