Is it gheebat for a person to tell his/her muslim community (or police) that a particular person is going to (nauzoobillah) kill or perform zina/rape another person/muslim? is it gheebat for a person to tell his/her muslim community that a particular person/Shia/Muslim is scamming/usurping another?
Any backbiting which prevents some one from serious crimes and sins or preventing others from such persons threat or danger is permissible and in some cases it can become obligated