A more precise term would be leaker, as somebody who commits a data breach or spillage (military term). Manning did release evidence that appears to constitute a potentially criminal act, however the quantity and material subject of information leaked greatly exceeded any single, or collection of, criminal acts at 750,000 unrelated items.
A whistleblower is a subset of leaking with the limited intention of exposing something specifically nefarious.
Whistleblowing only works if the people significantly higher up aren't supportive or complicit with the illegal actions.