Hamlet accidentally killed Polonius thinking that the latter was Claudius spying on him and his mother. That's why when Claudius asked for Polonius's whereabouts, Hamlet responded that perhaps Claudius should seek Polonius in hell because that's where Hamlet believed Polonius would end up. In a sense, Hamlet is trying to say that Claudius also deserves to go to hell for using his power at the expense of others, for being manipulative, and for being undeserving to the throne (killing King Hamlet).
