so it is not classed as backbiting if it a warning to the people from certain people etc, but remember once u have warned and they have understood- there would be no need to continue talking about someone, exposing them, this will become backbiting. e.g. i have 10 bad points about a deviant i want to warn you about them, u have understood he is a deviant after hearing the first 2 points hence there is no need for me tell u the ohter 8, but if i tell u the other 8 then i have done back biting,

and ALLAH knows best
may ALLAH save us from backbiting. ameen