In a situation where one is asked for information based on a legitimate need (such as for a shidduch) about a person whom he personally dislikes, the Chofetz Chaim rules that he may not divulge any information which would be considered Lashon Hara unless he can do so without feeling any personal pleasure; otherwise he has committed Lashon Hara.
[His proof to this is that the Mitzriyim were punished even though they fulfilled the Ratzon Hashem of Ger yihyeh zaracha… He argues that they were held responsible because they took pleasure at making the Bnei Yisrael suffer.]
Many Poskim disagree and are of the opinion that although he may not take pleasure at telling the information, he is still required to share it even if he can not help but enjoy telling. This would not be a violation of Lashon Hara, although it is clearly prohibited.
[The Ramban in Chumash says the Mitzriyim were punished because they were not compelled to be the ones to carry out the edict against the Bnei Yisrael.]