I'm writing a novel which centers around a serial killer and his victims. I'm having a lot of trouble deciding whether it belongs in the horror genre (it's a disturbing, horrific slasher, and a lot of the book works on horror and dread) or in crime (because a lot is about the good guys' hunt for the killer, and also there's no supernatural element).
I've read other fiction which seems similarly borderline to me (Silence of the Lambs by Thomas Harris, J.A. Konrath's first Jack Daniels book, and others), and on Amazon I indeed see these books listed in both categories. But I find it's important to have a single primary tag to focus on - for my elevator pitch, for query letters, for general promotion.
How can I tell which genre is better for my particular book? And, once I decide on one or the other as my 'main' genre, is there anything I need to do to fit better into the genre I've chosen, and make the book less borderline?