Betrayal is the greatest possible wrong that can be committed in the world of The Lion, the Witch, and the Wardrobe. The consequences of treachery are not limited to the information recounted by a spy or the tactical advantage he gives. Instead, traitors themselves are forfeit to the other side, and they can only be redeemed by the sacrifices of others.
Despite Edmund's treacherous alliance with the White Witch, he is always a good person at heart; he simply deludes himself briefly.
Edmund's own experience as a traitor teaches him to understand the mindset of people who violate ethical principles, making him better suited to dole out justice as an adult than his brother or sisters.