Since you went there:
Oxford dictionary says: "A person who informs on a person or organization engaged in an illicit activity."
What is illicit? : "Forbidden by law, rules, or custom."
Is what the cables actually revealed customary? Probably.
By the definition set forth in the law[1][2], but it's not just crimes. There is a list of specifically acceptably things.
1: https://www.gpo.gov/fdsys/pkg/STATUTE-103/pdf/STATUTE-103-Pg...
2: https://www.law.cornell.edu/uscode/text/5/2302
Since you went there:
Oxford dictionary says: "A person who informs on a person or organization engaged in an illicit activity."
What is illicit? : "Forbidden by law, rules, or custom."
Is what the cables actually revealed customary? Probably.