In the lion the witch and the wardrobe why does peter call Edmund a poisonous little beast?

No-one believed Lucy's initial tales of Narnia and the land she had somehow managed to get into through the back of the wardrobe. Edmund managed to follow Lucy into Narnia one day, but when the children both returned to their own land, he pretended that he just made the story up to amuse Lucy, and was just playing along with her. This upset Lucy for days - until all four children "fell into" Narnia.

The reason Peter called Edmund a "poisonous little beast" was that, when they all found their way into Narnia, Edmund accidentally gave away a clue that he had indeed been there. By doing so, he showed that he had let them all think Lucy was crazy and making up her stories of Narnia. That was why Peter was so angry with him.