Pathological lying is one of the hallmark characteristics of a narcissist, who does it out of a need to manipulate and maintain control. For the immature narcissist there is an essential emotional truth: lying is an expression of his (or her) mistrust of others; and his (or her) need to maintain a fragile sense of self at all costs. Being honest (and therefore vulnerable) terrifies the narcissist. Since his primary goal is to control others, through projection he constantly fears that others will try to control him. Thus, lying becomes the modus operandi through which he can maintain his superficial presentation of himself and keep people from learning the truth of who he really is. He never allows himself to be “pinned down”, or accountable. More lies are always necessary to cover up a previous lie. And typically, he even begins to believe his own lies and become outraged at any suggestion that he may be lying. Thus he becomes sincere in his lying and others may actually believe the lies because of the sincerity.
Hillary channeling her inner Nixon. “I am not a crook” :-)