Prompt: I would love to see Detective Childermass in pursuit of Serial Killer Lascelles. Lascelles is extremely twisted in that he is the stabby stabby kind of murderer. Let's him become more intimate with his victims. He may or may not be completely bonkers.

Childermass apprehends him, but he doesn't take him to the proper authorities, oh noooo, but keeps him prisoner at his place. So long as handcuffs are involved. :P
Childermass detests him but then becomes a bit obsessed.
It can be 19th C or Modern.

Fill: http://jsmn-kinkmeme.dreamwidth.org/1273.html?thread=644857#cmt644857