Iago in Verdi's Otello (1887) and John Claggart in Benjamin Britten's Billy Budd (1951) are both operatic villains. They do not have the stature of eponymous protagonists such as Verdi's Nabucco, who ruthlessly oppressed the Jews, Musorgsky's Boris Godunov, who allegedly ordered the assassination of the Tsarevitch Dimitri, or Verdi's Macbeth, who killed Duncan and ordered Banquo's death. Instead they resemble ‘secondary’ characters such as Don Pizarro in Beethoven's Fidelio (1814), who imprisons Florestan and intends to kill him, and Scarpia in Puccini's Tosca (1900), who tortures Cavaradossi and threatens to have him killed if Tosca does not submit to his sexual lust. But it is the evil nature of Iago and Claggart that drives their respective operas to their tragic conclusions. Iago's deceit and manipulation causes Otello to kill Desdemona and then himself, whereas Claggart's cunning results in the innocent Billy Budd being charged with mutiny and hung from the yard-arm. In contrast to Nabucco and Boris, who show remorse and repentance, or Macbeth, who is overcome with melancholy before being killed, neither Iago nor Claggart shows any signs of guilt for their malevolence (just like Pizarro or Scarpia). They embody the dark, destructive force in human nature; they remain mysterious and evil; and there will always be people like them to disturb and haunt us.

Iago and Claggart are examples of ‘psychopathic personality disorder’, now known as antisocial/dissocial personality disorder. The term is mired in confusion, and the concept has changed over time. There is both a legal definition in the Mental Health Act and a popular, non-medical connotation. Initially, ‘psychopathic personality disorder’ embraced the whole of what we now call personality disorder. It was only in 1980 that it was replaced in the (still current) psychiatric classification systems by the antisocial (DSM) and dissocial (ICD) types and included in the Cluster B disorders along with the borderline/emotionally unstable, histrionic and narcissistic ones.

Hervey Cleckley, an early pioneer in the study of psychopathic personality disorder, believed that these people were truly insane, afflicted by an illness almost as severe as psychosis. He called his classic book on the subject The Mask of Sanity (1941). This raises the question as to whether or not the disordered individual is responsible for his or her behaviour. Although antisocial and dissocial disorders are no longer thought of as an illness, the dilemma about responsibility remains.