Sadistic personality disorder is a personality disorder involving sadism which appeared in an appendix of the Diagnostic and Statistical Manual of Mental Disorders (DSM-III-R). The later versions of the DSM (DSM-IV, DSM-IV-TR and DSM-5) do not include it.
Sadism involves deriving pleasure through others undergoing discomfort or pain. The opponent-process theory explains the way in which individuals not only display, but also enjoy committing sadistic acts.[clarification needed] Individuals possessing sadistic personalities tend to display recurrent aggression, abusive and cruel behavior. Sadism can also include the use of emotional cruelty, purposefully manipulating others through the use of fear, and a preoccupation with violence.
|Spineless sadism||Including avoidant features||Basically insecure, bogus, and cowardly; venomous dominance and cruelty is counterphobic; weakness counteracted by group support; public swaggering; selects powerless scapegoats.|
|Tyrannical sadism||Including negativistic features||Relishes menacing and brutalizing others, forcing them to cower and submit; verbally cutting and scathing, accusatory and destructive; intentionally surly, abusive, inhumane, unmerciful.|
|Enforcing sadism||Including compulsive features||Hostility sublimated in the "public interest," cops, "bossy" supervisors, deans, judges; possesses the "right" to be pitiless, merciless, coarse, and barbarous; task is to control and punish, to search out rule breakers.|
|Explosive sadism||Including borderline features||Unpredictably precipitous outbursts and fury; uncontrollable rage and fearsome attacks; feelings of humiliation are pent-up and discharged; subsequently contrite.|
Sadistic personality disorder has been found to occur frequently in unison with other personality disorders. Studies have also found that sadistic personality disorder is the personality disorder with the highest level of comorbidity to other types of psychopathological disorders. In contrast, sadism has also been found in patients who do not display any or other forms of psychopathic disorders. One personality disorder that is often found to occur alongside sadistic personality disorder is conduct disorder, not an adult disorder but one of childhood and adolescence. Studies have found other types of illnesses, such as alcoholism, to have a high rate of comorbidity with sadistic personality disorder.
Researchers have had some level of difficulty distinguishing sadistic personality disorder from other forms of personality disorders due to its high level of comorbidity with other disorders.
Numerous theorists and clinicians introduced sadistic personality disorder to the DSM in 1987 and it was placed in the DSM-III-R as a way to facilitate further systematic clinical study and research. It was proposed to be included because of adults who possessed sadistic personality traits but were not being labeled, even though their victims were being labeled with a self-defeating personality disorder. Theorists like Theodore Millon wanted to generate further study on SPD, and so proposed it to the DSM-IV Personality Disorder Work Group, who rejected it. Millon writes that "Physically abusive, sadistic personalities are most often male, and it was felt that any such diagnosis might have the paradoxical effect of legally excusing cruel behavior." 
There is renewed interest in studying sadism as a personality trait. Sadism joins with subclinical psychopathy, narcissism, and Machiavellianism to form the so-called "dark tetrad" of personality.
A research has been conducted where neuroscientists put two groups of subjects into an FMRI : normal people and aggressive/sadistic bullies, the first group is a healthy and the second is diagnosed with aggressive conduct disorders.These two groups where tested in an FMRI scanner by viewing pictures of people being hurt and harmed, the healthy controls showed a strong neural activation in the right mid anterior insula and the dorsal anterior cingulate cortices (brain areas associated with pain,empathy,salience and many other cognitive processes), while the aggressive subjects showed a lack of activation in the same brain areas mentioned above along with the medial prefrontal cortex and temporo-parietal junction (brain regions known to importantly contribute in self awareness, self regulation, judgement, affect, self-other distinction and many other functions), strong activation in the amygdala and ventral striatum are also reported in the aggressive subjects (brain areas that process emotions, rewards, motivation among many other processes) which may explain why bullies can feel pleasure while watching people in pain or distress, this suggests that aggressive bullies may have disrupted neural empathic responses in ways that increase aggression , this could explain why they enjoy seeing others in pain
Other data has shown that an area of the brain called the supplementary motor area or SMA (brain region that controls motor functions in relation to cognition and perception,behavioral reactions to sensory stimuli and many other functions) may be deficient/disrupted in sadists and bullies, this could also explain their abnormal and inappropriate reactions to other people's pain and suffering