Lucien Burgess and DCI Luther in “Unfinished Business”

It's a kink, a sex thing. Many many women fantasize about being tied up, strangled, raped, eaten.

DCI Luther

You think so?

Oh, yeah. The more depraved the killer, the more women fawn on him. It's called 'hybristophilia'.

DCI Luther

'Sexual arousal from thoughts of violence.'

It's his willingness to murder that attracts her. His murder becomes hers.