I've read a small handful of true crime books lately, and this one seems different from the rest. It's not structured as a mystery might be, for one. This is basically because, until human remains were found to be blocking a drain, no one in the community (neither police nor civilians) was aware that a serial killer had murdered at least 15 men. No one had the faintest idea, and they wouldn't have any clue of the extent of the slaughter either, had Nilsen not openly confessed the minute he was taken in for questioning. He just seems an extremely odd individual, and it's clear that, even after all their interaction, Masters can't figure him out either. 

It's hard to read this and not come away with the conclusion that something was very wrong with Nilsen's brain, except that is something that might be said for all serial killers. I think the most interesting thing for me here was an early chapter on the history of mental illness in Nilsen's family... it's significant and well-recorded and seems to have lurked in the foreground for generations until Des Nilsen came along and started doing terribly violent and inexplicable things. I'm not sure how else you can describe necrophilia other than "inexplicable," to be honest, but frankly I am glad I don't understand it, and I'm not sure I want to spend anymore time trying.