Calling someone an idiot would very rarely be considered "grossly offensive or of an indecent, obscene or menacing character;". Some words or actions are menacing, for example if someone said they wanted to rape your child. And some behaviour is inextricably linked with violence such as racism. Obviously there is nuance and context here, and the police don't always get it right. But threatening language absolutely does exist. Sometimes I think we need to be more explicit about what that context actually is and social networks tend to have a wide audience with different expectations.