Possessing glibness or superficial charm doesn't mean you merely have a charming personality, in this context. It means you know how to present yourself in a situation to create a desired reaction. That is, believability. It is a conscious choice to manipulate someone through the force of your personality. Thus, one can be charming and influential, without necessarily being a psychopath. But a psychopath will consciously choose such personality characteristics to manipulate someone's impression of them to achieve their aim. This might be murder or it might be getting votes to achieve a position of power. This glibness and charm thus becomes predatory and not genuine.