Who poured poison in the king's ear, and why?

Prince Hamlet's uncle Claudius committed regicide (and fratricide) by pouring poison into the king's ear while he was sleeping in his orchard. Claudius assassinated his brother for a variety of reasons. He was motivated to attain the power, authority, and wealth that comes along with being king of Denmark. In addition to his lust for Denmark's crown, Claudius also desired to be with brother's wife, Gertrude. Claudius murdered King Hamlet before the start of the play, and Prince Hamlet discovers the truth about his father's death in act 1, scene 5. When Hamlet is visited by his father's ghost, the Ghost tells him that he did not die from a poisonous snake bite, as people previously thought: Claudius poured a vial of henbane poison into his ear while he was sleeping in the orchard. The Ghost then instructs Hamlet to avenge his death by murdering Claudius. Hamlet initially questions whether or not the Ghost is telling the truth and hesitates to get revenge for the majority of the play.