A traitor is someone who betrays. Being a traitor requires first relationship. It can be as close as best friends; a husband and wife; an officemate; or the relationship can be as far as the Philippines is to a Filipino; or as an employee is to a company.
And because that person is a friend or an employee or of a particular nationality he or she is expected to act in the interest of the relationship which makes the proverbial knife in the back by this person to be one of the most painful assaults anyone can inflict another.