https://www.sciencedaily.com/releases/2018/09/180926110841.htm

Both world history and everyday life are full of examples of people acting ruthlessly, maliciously, or selfishly. In psychology as well as in everyday language, we have diverse names for the various dark tendencies human may have, most prominently psychopathy (lack of empathy), narcissism (excessive self-absorption), and Machiavellianism (the belief that the ends justify the means), the so-called 'dark triad', along with many others such as egoism, sadism, or spitefulness.

Although at first glance there appear to be noteworthy differences between these traits -- and it may seem more 'acceptable' to be an egoist than a psychopath -- new research shows that all dark aspects of human personality are very closely linked and are based on the same tendency. That is, most dark traits can be understood as flavoured manifestations of a single common underlying disposition: The dark core of personality. In practice, this implies that if you have a tendency to show one of these dark personality traits, you are also more likely to have a strong tendency to display one or more of the others.