Evil should not be uttered in public

Allah does not like that the evil should be uttered in public except by him who has been wronged. And Allah is Ever All-Hearer, All-Knower. 

[Qur'aan 4:148]

فقال بعضهم : معنى ذلك : لا يحب الله - تعالى ذكره - أن يجهر أحدنا بالدعاء على أحد ، وذلك عندهم هو " الجهر بالسوء إلا من ظلم" 

A group of scholars said that the meaning of that is that Allah does not like that anyone of us makes duaa' (supplication) against anyone. And that is the meaning, in their opinion, of the portion which reads "evil should not be uttered in public except by him who has been wronged."

يقول : إلا من ظلم فيدعو على ظالمه ، فإن الله جل ثناؤه لا يكره له ذلك ، لأنه قد رخص له في ذلك

They say the exception is the one who has been wronged. He has the right to make supplication against the person who wronged him. Allah does not dislike that because he has given him dispensation regarding that.