Splitting (also called black-and-white thinking or all-or-nothing thinking) is the failure in a person's thinking to bring together the dichotomy of both positive and negative qualities of the self and others into a cohesive, realistic whole. It is a common defense mechanism used by many people. The individual tends to think in extremes (i.e., an individual's actions and motivations are all good or all bad with no middle ground).from Wikipedia
People matching the diagnostic criteria for narcissistic personality disorder also use splitting as a central defence mechanism. Most often the narcissist does this as an attempt to stabilize their sense of self positivity in order to preserve their self-esteem, by perceiving themselves as purely upright or admirable and others who do not conform to their will or values as purely wicked or contemptible. Given "the narcissist's perverse sense of entitlement and splitting ... [s]he can be equally geared, psychologically and practically, towards the promotion and towards the demise of a certain collectively beneficial project".
The cognitive habit of splitting also implies the use of other related defence mechanisms, namely idealization and devaluation, which are preventative attitudes or reactions to narcissistic rage and narcissistic injury.