Disorder(s): Narcissistic personality disorder, sadistic personality disorder, passive-aggressive personality disorder, possible antisocial personality disorder OR psychopathy
Positive Traits: Extremely high intelligence, possesses ability to formulate and carry out sophisticated (one may even Machiavellian) plans, strong ability to understand and predict human behavior, strong strategical and logical intelligence
Negative Traits: Disregard for the safety of oneself and others, inability to feel remorse or guilt for actions, inability to sympathize with others, sadistic tendencies (mostly regarding the psychological pain of others), inability to cope with boredom properly, manipulative and deceitful, strong desire for chaos and disorder, extremely large ego, disregard for authority, indifference to the wants and needs of others, prone to obsession, tendency to identify and exploit weaknesses of others
Notes: Jim Moriarty is psychologically similar to Sherlock in many ways. They share the same personality type (INTJ), and a lot of the same positive and negative traits. However, the difference between the two is that Sherlock copes with his boredom in ways that benefit others, whereas Jim Moriarty copes with his boredom in ways that are destructive to not only others, but himself.
Moriarty appears to see no point in exercising self-control or following morals. He is somewhat of a hedonist. He is intelligent enough to know the consequences of his actions and the fact that his actions are wrong, however, he does not care. When it comes to morality, he displays pure apathy. It seems that the only thing he cares about is indulging himself, and in carefully constructing a satisfying self image of himself.
He seems to find either indifference or joy in others pain (he is most likely a sadist), and he is also extremely narcissistic. He believes, like Sherlock, that he is above everyone else, and also considers, unlike Sherlock, normal people to be completely undeserving of any and all consideration. That may be one of the reasons why he does not care about killing them; he finds no value in the lives of “ordinary” people.
Thus, when he discovers Sherlock Holmes, a man of equal intelligence to him, he simultaneously becomes obsessed with not only killing Sherlock, but completely destroying him, even if it means killing himself in the process, as well as considering Sherlock to be a distraction, a form of entertainment; the best challenge he has ever found.
I also believe that Moriarty must have suffered through or witnessed some sort of abuse or trauma in his childhood, most likely extreme physical or sexual abuse, not just once, but often, and from people he trusted. It’s clear that he must have been exposed to violence at an early age, in such a manner that it became second nature to him. If there were ever people that he trusted or even loved, it is likely that they either died or abandoned him. This would explain some of his apparent philosophies (“People have died—” “That’s what people DO!”) and even his indifference to the pain of others; he believes that pain and death is an inevitable part of life, something that cannot be avoided and instead should be mastered or controlled, which he has attempted to do.
Moriarty and Sherlock have a fascinating relationship. It appears that Sherlock Holmes is the only man Moriarty has ever considered his equal. He displays mostly apathy to other “ordinary” people; either ignoring them completely or using them as pawns in his plans. He even refers to John Watson as Sherlock’s “pet”, and he appears to be slightly resentful of him as well. It’s really not that he is jealous of Sherlock and John’s relationship, but more that he doesn’t want his relationship with Sherlock to be, in his opinion, downgraded to that level. If you noticed in the tea party scene in The Reichenbach Fall, Moriarty refuses to sit in John Watson’s chair when Sherlock gestures to it with his violin bow. That could, however, just be a subtle display of dominance.
On the subject of dominance, it is interesting to note that both Sherlock and Moriarty’s body language regularly suggests dominance (Sherlock: walking with hands behind the back, sitting with fingertips touching or in the prayer position; Moriarty: walks with hands in his pockets). Neither Mycroft Holmes nor Irene Adler displays dominant body language, and yet they are both more obsessed with power play than any of the other characters.
I think Jim Moriarty is a sociopath. I am not entirely sure. He displays most of the traits of antisocial personality disorder, but a few are exempt. If he had had conduct disorder as a child, which is seen as a precursor to antisocial personality disorder, then he may be sociopath. Whether or not he is a psychopath is also questionable; the lines between antisocial personality disorder and the disorder known as psychopathy are often blurred or skewed, but from my interpretation, I believe that Jim Moriarty fits criteria for both. Normally, this would point to psychopathy (as most psychopaths fit the criteria for ASPD well, except to a more heightened extent, whereas on the other hand, most sociopaths do not fit the criteria for psychopathy), but there are other factors that contradict this, and because of this, whether or not Jim Moriarty is a sociopath, psychopath, or neither shall remain open to discussion.