Venting is not permissible and it is backbiting.

Does complaining about someone fall into backbiting? Or does it have limits? Sometimes ppl come to my mom to complain abt someone.. Usually they do it to let out their feelings. People also narrate things that has happened in their homes.. Arguments etc. The Intention is not for fun or to laugh at someone. But usually we end up learning about someone’s bad qualities

Backbiting | July 13, 2017 6:40 pm
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I laugh at someone because he looks funny when he speaks.Or, someone when they speak their accent is funny. Or, when I look at video and the man is bald,I laugh. By laugh,I don’t mean laughing out loud. By laugh,I mean smiling or slightly twisting your lips only. Because I am trying my best not to laugh. Is this backbiting (if done in secret) or insult?
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Is it backbiting if i inform a person of a habit or an evil of someone that could actually harm him ? For example if i tell my sister or a friend to not believe everything mr. Xyz says because he lies ,then will it be considered backbiting? If it is then how are we to warn our beloved one of treacherous or evil people ? And what if the evil person is in family ?