Anna Dressed in Blood by Kendare Blake

So did his father before him, until he was gruesomely murdered by a ghost he sought to kill. Now, armed with his father's mysterious and deadly athame, Cas travels the country with his kitchen-witch mother and their spirit-sniffing cat. Together they follow legends and local lore, trying to keep up with the murderous dead—keeping pesky things like the future and friends at bay.

When they arrive in a new town in search of a ghost the locals call Anna Dressed in Blood, Cas doesn't expect anything outside of the ordinary: track, hunt, kill. What he finds instead is a girl entangled in curses and rage, a ghost like he's never faced before. She still wears the dress she wore on the day of her brutal murder in 1958: once white, now stained red and dripping with blood. Since her death, Anna has killed any and every person who has dared to step into the deserted Victorian she used to call home.

But she, for whatever reason, spares Cas's life.