Brian Masters, biographer of the notorious murderer Dennis Nilsen, explains why – uncomfortable though it is – there is much to learn from studying his behaviour

Watching David Tennant’s exceptional performance as the serial killer Dennis Nilsen in the three-part ITV drama Des last week, took me back over a quarter of a century to the winter evening when I read through Nilsen’s letters from prison to his biographer, Brian Masters. Masters had donated them to the Royal Society of Literature, where I worked, to be sold at a fundraising auction at Sotheby’s – and they were disturbing. Every word Nilsen wrote spat out fury and contempt for everything, except himself: ironic, as he had strangled and dismembered at least 12 men.

The letters were written in black biro, pressed down very hard, the words crammed on to the pages, leaving no breathing space. When Masters showed a sample of the writing to a graphologist, not saying whose it was, she was alarmed, and warned him to steer clear. But he persevered, not just exchanging letters with Nilsen but visiting him regularly in prison.