Laertes truly doesn’t know how he could have started out so certain that the king was who he needed to destroy and then become – well it’s not like they’re allies, but they’ve reached some sort of détente. He just wants to do the right thing, and for that it’s true he needs all the information, which he could not easily get from a corpse, so he does his best to believe he’s been constant to his purpose.

Something feels wrong about transferring all his ire to Hamlet, when Claudius is still the one who allowed things to get this bad and then waited for a proxy to handle the problem, but as Laertes cannot let the prince go unpunished, he will just have to be that proxy.