If it were done when ’tis done, then ’twere well
It were done quickly: if the assassination
Could trammel up the consequence, and catch
With his surcease success; that but this blow
Might be the be-all and the end-all here,
But here, upon this bank and shoal of time,
We’d jump the life to come. But in these cases
We still have judgment here; that we but teach
Bloody instructions, which being taught, return
To plague the inventor: this even-handed justice
Commends the ingredients of our poisoned chalice
To our own lips.

– William Shakespeare

Macbeth, Act 1, Scene 7. In Macbeth’s second soliloquy he is plagued with self-doubt over whether he should kill King Duncan and the consequences for him if he does. If he carries out the assassination, he believes that it must be done quickly. But he fears the retribution he will face, both in this world and the next. The killing might not bring an end to the whole affair, his violent deed could return to haunt him and trigger a cycle of violence that could destroy him. Using the metaphor of a “poisoned chalice,” Macbeth foresees that justice could ensure that he drink from the same cup that he himself poisoned. In other words he may end up dead like Duncan. The dark imagery of “bloody instructions” and “poisoned chalice” suggests that Macbeth knows the King’s murder would unleash a dark and evil period in Scotland. And it does. The passage foreshadows that and Macbeth’s eventual death. An ocean metaphor is used to compare time to a bank or shoal (a shallow part of the ocean). Shakespeare uses another metaphor to compare the murder to bloody instructions being taught.