Why is rosencrantz’s repeated line “he murdered us,” significant?

The line has two meanings.

First, Rosencrantz is referencing the questions game. Rosencrantz says that Hamlet "murdered" them at the game, meaning that he easily beat them.

The line, of course, has a deeper, ironic meaning -- one that the audience would recognize. Rosencrantz and Guildenstern will die, thanks to Hamlet -- making Rosencrantz's line an example of both foreshadowing and irony.