It's an old and massively overused cliché, but still very reliable: reality is far more astounding, far more unbelievable, and far more disturbing than any fictional story could ever be! Of course, this is most applicable to the film/TV adaptations of true-crime stories, like biographies or semi-documentaries revolving around serial killers.
I've read countless biographies and factual reports on real-life serial killers, but the case of Dennis "Des" Nilsen undoubtedly is the one of the most uncanny ones in the history of human darkness. It's nearly impossible to believe that this seemingly dull and miserable looking man actually was such a stoic, relentless monster and, even more unlikely, capable of misleading the authorities before and during the discovery of his atrocities. Between 1978 and 1983, Des - a nickname he mainly just uses himself - murdered between 8 and 15 young men and boys. More disturbingly even, he kept the corpses near him for longer periods, to talk to or watch TV with, before boiling the parts or hide them underneath the floors of his apartments.
Particularly the first half of this mini-series is brilliantly tense, grim and compelling. Nilsen, indifferent about the fact that he finally has been arrested, narrates his atrocious crimes as if he's reading out a groceries' list. In practically every review available on the Internet, David Tennant is extendedly endorsed for his portrayal of Nilsen, and it's more than justified. The resemblance is eerie, but his emotionless and stoic body language is even more impressive. The second half, mainly revolving about the interactions between Nilsen and his biographer, and the trial, is far less overwhelming, but nevertheless very competent television-making.
