A Personality Assessment of Online Haters

Narcissism: a personality disorder where there is a persistent pattern of grandiosity, fantasies of unlimited power or importance, and the need for admiration or special treatment (gosh, that sounds familiar)

Psychopathy: a neuropsychiatric disorder marked by deficient emotional responses, lack of empathy, and poor behavioral controls, commonly resulting in persistent antisocial deviance and criminal behavior (also familiar)

Machiavellianism: a personality trait characterized by interpersonal manipulation and associated with specific patterns of emotional and social cognition skills (hm, there seems to be a pattern here) 

Poland

A small study of people in Poland who posted online hate regarding the 2018 Olympics suggested that those people tended to score higher than usual on one personality assessment. That assessment, the Dark Triad questionnaire, assesses narcissism, psychopathy and Machiavellianism. The researchers found that hate commenters scored high for psychopathy but not other traits associated with disruptive behavior.

One of the study's limitations is that it was based on characteristics of sports fans, not the general population. Also, the data may not be generalizable to other, more diverse societies than Poland's. This kind of research needs to be expanded and verified. However, such research could help shed light on what is part of the forces that are tearing American society apart and what is leading millions of people to support autocracy and distrust of democracy and the rule of law.

The authors concluded: "Considering a reported increase in online hating (Blaya, 2019), predictions are that online hating behavior will become even more and more severe. Results of the present study are one of the first steps in broadening our understanding who the online haters are, which, in turn, may help identifying the best strategies for psychological interventions for haters, and creating counter-hating strategies."

One can only wonder what America's former Hater-in-Chief would score on the Dark Triad and whether any counter-hating strategy would ever faze that ferocious beast.

Germany

A 2021 paper indicated that predictors for participation in hate speech include being exposed to it. The researchers commented on the the correlation between personality and online hate: 

The association between users’ personal characteristics and uncivil participation was overall relatively weak. In line with prior research about dark personalities’ uncivil behavior, we found that psychopathy and Machiavellianism increased self-reported uncivil participation (RQ1), whereas narcissism did not. When participants scored one scale point above average on the dark triad, they were roughly 20% more likely to engage in uncivil participation. This modest effect size could also explain why other studies with fewer participants failed to find a link between the dark triad and uncivil participation intentions.

So, maybe doing online hate begets more online hate. If so, it's probably best to avoid it and prevent it when possible.