Has Laertes realized that the shit is about to hit the fan? Did he clock that Gertrude has just drunk poison? Does he know what’s about to happen to her or was he somehow busy with between round adjustments? He knows Claudius was going to poison Hamlet’s drink so theoretically he knows that chaos is about to break out. If he wants to kill Hamlet, it has to happen quickly. He needs to get it done before he loses the chance. It explains why he steps out of the bounds of the game to do it. Perhaps questioning his conscience makes him all the fiercer and more determined.