Responsible people blow the whistle when they believe more harm than good will occur if the whistleblower stays silent. A virtuous whistleblower acts in an ethical manner if she truly believes a responsibility exists to protect the public interest. Such a person is willing to accept the consequences of her actions. i.e., she is accountable for her actions.
Loyalty is a powerful ethical value and may inhibit a would-be-whistleblower from coming forward. However, there are numerous examples of where loyalty trumped higher ethical values, such as honesty and integrity, with the result being that financial fraud was not disclosed with devastating results for shareholders.