You are forbidden to say that a person did something improper, even if you are motivated by kinaas ha’emes – that is, you are agitated about the transgression and insistent that the truth be known.  Although your words might be meant l’shaim shomayim and are without malice, they still constitute loshon hora.

You are allowed to relate details of improper action only if your intention is constructive.  ~ Guard Your Tongue