Abu Hurairah reported Allah’s Messenger (may Allah bless and greet him) as saying: The servant (who conceals) the faults of others in this world, Allah would conceal his faults on the Day of Resurrection.
Islam is very much against character assassination and even against hurting a person in any way. That is why it considers talking ill about a person in his absence to be backbiting, which includes all those acts going a long way in degrading a person. But this does not mean that one should provide a covering to the faults and/or weaknesses of a person and should not let them be known to others. What this implies is that one should avoid publicising the faults and shortcomings of others and thus degrade them in society. A Muslim is the brother and thus well-wisher of another Muslim and as such the latter deserves the former’s help, cooperation and assistance. In case there is some genuine necessity, one should readily disclose them and if one hesitates in doing so one would be answerable for it on the Day of Resurrection.
If a person has committed any cognisable offence and another person knows it, the latter should inform the authorities about that. If a ruler or a person in power commits evil deeds, it should be publicised in the greater interest of society as concealment of these would lead to the ruin of the Ummah. If a man has to marry his daughter and asks another person about the character of one with whom he intends to marry her, it is duty of the person whose counsel has been sought for to give him correct information about the person with whom he wants to marry his daughter.
It should however be kept in mind that an oppressed person is allowed to talk ill against the oppressor even in his presence in order to tell people that he is being oppressed and they should manage to get the wrong redressed for him.