Having ascertained that the negative information is accurate, that the perpetrator was not justified, and that reproof has proven unsuccessful (thus making it clear that there is a legitimate need to relate the information), one final condition must be met before the information can be conveyed. The speaker must be certain that his sole intent is l’toeles, for a constructive purpose. However, if he knows that in his heart he is pleased to cast the perpetrator in a bad light, he is not permitted to speak – even though the purpose cnanot be accomplished any other way.
It is lowly to enjoy speaking negatively – even for a constructive purpose.
This is especially challenging when one is required to speak about a person with whom he does not get along. It is necessary to first eradicate any negative feelings towards that person, and only then can the negative information be spoken.
Sefer Chofetz Chaim, 18 Cheshvan, page 144