Is warning people about a man who is known to commit evil openly regarded as backbiting?

If the situation is as described and he mentions him so as to
warn people against his evil so that those who do not know about him will
not be deceived by him, it is permissible, but if it is only done for fun
and to pass the time and so on then it is not permissible. 

And Allaah is the Source of strength. May Allaah send
blessings and peace upon our Prophet Muhammad and his family and companions.
End quote. 

Standing Committee for Academic Research and Issuing Fatwas