The psychological condition known as Antisocial Personality Disorder, otherwise called sociopathy, is mostly a personality disorder in which the person has a pattern of disrespect for the rights of others, as well as a pattern of violating others' rights (Psychology Today, "Diagnosis Dictionary: Antisocial Personality Disorder," psychologytoday.com). Sociopathy is similar to psychopathy, but specialists define sociopathy as "something severely wrong with one's conscience," while psychopathy is a "complete lack of conscience" (Psychology Today). While not all of the main characters can be considered sociopaths, Demetrius certainly can be, and due to external circumstances, Helena does demonstrate at least one sociopathic symptom.
We especially see how Demetrius can be considered sociopathic with respect to his treatment of Helena. For absolutely no known reason, he transfers his affection from Helena to Hermia, even though he was already engaged to Helena. An engagement is considered a social contract. Having complete disregard for his social contract helps peg Demetrius as a sociopath. We further see sociopathic behavior in his treatment of Helena after she follows him into the woods. He threatens her with physical violence, even rape, showing us that he has complete disregard for her rights as a person. We see him threaten her with violence when, after she begs him to treat her as a spaniel, he replies, "Tempt not too much the hatred of my spirit; / For I am sick when I do look on thee" (II.i.215-126). We further see him threaten her with rape in his next lines:
You do impeach your modesty too much
To leave the city and commit yourself
Into the hands of one that loves you not;
To trust the opportunity of night,
With the rich worth of your virginity. (218-223)
The references to her modesty and virginity are a clear threat upon her maidenhood and a clear threat of rape. Demetrius's threats of violence show that he lacks a conscience and also lacks respect for social codes and for Helena as a person, showing us that he is indeed sociopathic.
While I would hesitate to call Helena, or any of the other characters sociopathic, we do see one instance in which Helena forgets moral codes and the rights of others. In the very first scene, she decides to betray her best friend's secret plans to escape with Lysander into the woods and out of Athens so that they can marry. Her motive for her decision of betrayal is that she may be gratified with just a simple "thank you" from Demetrius, as we see in her lines, "...and for this intelligence / If I have thanks, it is a dear expense" (I.i.253-254). While Helena does disregard moral codes for a moment, it cannot really be said that she has a questionable conscience as this is the only instance of disregard that we see in her. Therefore, it cannot be said that she is sociopathic. Instead, one must consider the external circumstances that have led her to make this decision, which is Demetrius's own abusive, unconscionable behavior.