Betrayer. Blabbermouth. Snitch. Tattletale. All the synonyms for whistleblower have negative connotations yet whistleblowing is, by definition, the act of exposing wrongdoing.
Whistleblowers often suffer bad reputations because while, on the one hand, they are building a foundation of truth, on the other hand, their exposed truth likely results in a different piece of our society crumbling.