A traitor says one thing but does another. If you promise a friend you'll keep his secret, but instead you blab it to everyone, you're a traitor.
The word origin says it all: traditorem is the Latin word for "betrayer." Liar? Yes. Backstabber? Yes. True friend? Heavens, no. Traitors betray the trust of those who have faith in them or believe their promises. Traitor also applies to a person who betrays his country by committing treason: turning against his own government, perhaps by selling secret information.
