Hamlet opens with poison and closes with poison. What is the literal and metaphorical use of posion throughout the play?
The literal use is, of course, to kill people. Most importantly, at the beginning, the King, Hamlet's father. Claudius uses poison to murder him and take over the kingdom. Then, Claudius and Laertes use poison for the final death plot. Claudius puts poison in the infamous wine glass, and Laertes puts poison on the tip of his rapier. Unfortunately for Claudius, his plan didn't go very well; the poison was also his own death-token.
Figuratively and metaphorically, we see how greed poisons and how Hamlet's quest for truth poisoned. Greed poisons Claudius; it prompts him to murder and to plot Hamlet's death while manipulating everyone around him. Everyone he plots with ends up dead, poisoned by his greedy touch: Polonius, Rosencrantz and Guildenstern, Gertrude, Ophelia, Laertes.
Hamlet is poisoned by his quest for and truth and revenge. He alters his behavior to investigate without restraint and thereby alienates those he cares about. He rejects Ophelia and murders her beloved father. She becomes mad and falls to a drowning death during which Gertrude reports she had no sense of her danger.
He rages against his mother, baffling her and turning her more toward Claudius. He scorns Rosencrantz and Guildenstern; he master-minds their murder then returns to Denmark to take revenge against Claudius. Hamlet's quest for revenge was a figurative, metaphorical form of poison that slowly ate away at him and affected many characters in the play, leading to Denmark's disastrous conclusion and Norway's unintended triumph.
Poison would be a figurative metaphor for moral corruption: a poisoned person dies from the damage inside themselves but may not show signs on the outside of their body.
In Hamlet's feudal society, men fought for their own honor in duels and fought for the honor of their king in wars. Fighting skills were considered very important. Medieval nobility were often violent warlords defending and attacking each others' territory.
So there were two skills that every nobleman was supposed to excel at, horsemanship and swordsmanship. By contrast, archery was not practiced by 'gentlemen' because archers were generally people from the bottom end of society.
In medieval Europe there was a strict and brutal code of honor called Chivalry (from the French 'Chevalier' meaning 'horseman' or 'knight'). A chivalrous knight was brave and true. The worst thing for a knight was to be thought a coward. It was much better to die than to be a coward.
So poison is completely unchivalrous. Poison is the weapon of a coward. A man who poisons is a liar and a cheat. Poisoning is the act of a man with no honor. And, in the days before science, poison was a dark and mysterious thing. It was devilish and monstrous magical potion. Using poison was a dark art! So, a poisoner is a devilish coward who has no truth or honor and who will go to hell.