Verified answer

I assume you are talking about Hamlet. This should be in the books or play section.

Laertes had heard the news that his father, Polonius, has been murdered and he wanted to make Claudius answer for what happened. He seems to have heard that Claudius himself had Polonius killed since he threatened Claudius with his sword until Claudius told him it was Hamlet's doing. He wanted revenge for his father's murder and if the king had done it as it can be guessed that's what he assumed, then he would have wanted to take the crown from Claudius along with Claudius's life.