One of the classic differences between villains and heroes are that the heroes’ methods are perfectly acceptable when applied against the hero or his enemies. The villains’ methods, however, are only acceptable to the villain when applied against the villain’s enemies. Applied against the villain and suddenly things are no good, purple. Suddenly the villains must be protected from their own methods even though those methods were justified when used against people like Joe or Sarah.
That’s how you can detect the difference ethics and lack of ethics.
This is one of those Rule of Thumb (RoT) things, that seems so obvious when you finally read, that you wish you had thought of it yourself (as I do) as it help crystalize thinking on ethics.
Update: Minor correction.