Why was Peter mad at Edmund?

There are a number of points where Peter gets mad at Edmund in these chapters. Edmund consistently picks on Lucy. Edmund questions the faun's integrity. I think the biggest reason is when suddenly, Lucy realizes that Edmund has disappeared. Everyone rushes out into the night, but they are blinded by the snow. Peter insists that they form search parties, but Mr. Beaver tells them that Edmund has most likely gone to the White Witch with the news about Aslan and their meeting at the Stone Table: "He has betrayed us all." Mr. Beaver asks whether Edmund has been in Narnia before, and Lucy whispers that he has.