Which plot event accounts for the change in Laertes's character in Act IV of Hamlet?

Laertes changes from a grieving son into someone bent on revenge after he learns that Hamlet killed his father, Polonius. He returns to Denmark and confronts Claudius, ready to act against Hamlet. The performance of The Murder of Gonzago is connected to Hamlet’s effort to test Claudius, not Laertes’s change in character.

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