Armchair psychiatric diagnoses of real people can probably be classified as, in general, a bad thing. Armchair psychiatric diagnoses of fictional characters, though: That's pretty much just awesome.
Case in point: The team of researchers at Toulouse University Hospital in France who not only make a pretty good, DSM-based case for the mental illness behind everybody's favorite black-clad evil overlord—they actually took the time to do the diagnosis right, publishing in the journal Psychiatry Research
'He presented impulsivity and difficulty controlling his anger and alternated between idealisation and devaluation (of his Jedi mentors). Permanently afraid of losing his wife, he made frantic efforts to avoid her abandonment and went as far as betraying his former Jedi companions. He also experienced two dissociative episodes secondary to stressful events. One occurred after his mother's death, when he exterminated a whole tribe of Tuskan people, while the other one took place just after he turned to the dark side. He slaughtered all the Jedi younglings before voicing paranoid thoughts concerning his former mentor and his wife. Finally, the films depicted his quest to find himself, and his uncertainties about who he was. Turning to the dark side and changing his name could be interpreted as a sign of identity disturbance.'
Sadly, I'm not sure I buy their argument that publicizing Darth Vader as a BPD sufferer is going to do anything to take away the stigma of mental illness.
British Psychological Society Research Digest: Does Darth Vader meet the diagnostic criteria for Borderline Personality Disorder?