“We have scorched the snake not killed it.”
The word snake is a metaphor for Banquo and Fleance. A snake is usually portrayed as being sly and venomous. This means that Macbeth who is says this is making out that they are sly and venomous. This is reinforced by a sibilance “scorched the snake” which makes the line seem more controlling and powerful. We is a pronoun, this is careful manipulation by Macbeth. He is saying instead of himself killing Banquo he is elevating his wife to the same level. This line uses imagery in the scorching of the snake.