p(Author is a sociopath | Author chose to RP as Voldemort specifically) > p(Author is a sociopath | Author went with a different pseudonym) is my basic assertion here. People who roleplay sociopaths are more likely to be sociopaths - roleplaying Voldemort is a safe outlet for that tendency.
That the author is writing Voldemort also seems like evidence for the hypothesis that the author agrees with Voldemort (I'd assume possibly not to that extreme, but who knows). Much the same as everyone assumes that the author behind shokwave agrees with shokwave's writing...
Sure, roleplaying as Voldemort may be evidence for sociopathy, but if I had to estimate how much evidence, I'd call it epsilon. Roleplaying, and humour, is fun. And fun is tempting, especially on the internet.
From Mike Darwn's Chronopause, an essay titled "Would You Like Another Plate of This?", discussing people's attitudes to life:
Conclusion, graphs, and references in article. As usual, I recommend reading Chronopause.com as Darwin has many good articles; to quickly link a few: