I'm pretty sure I found this again several years ago, and learned it was a series, but I first read it when I was in elementary school (although I think it was supposed to be for older readers). The main character is a samurai in Japan. I don't know if he'd been sent by someone, or if he was doing it on his own, but the village that the story is set in has had a rash of killings where people have been found with their throats torn out, with the eerie yowl of a cat heard from a distance. I think the actions in the book happened during winter because I have a vague memory of one of the fights happening in the middle of a snow storm, maybe connected with the assumption of supernatural events since the victims were apparently killed when no one was around according to the tracks.
It turns out that the killer is a former samurai, known to the protagonist, maybe his brother, who'd had his throat severely damaged, causing him to emit the catlike yowl. He was killing people with a grappling hook, explaining the throats torn out with no tracks nearby. When he is found out, the protagonist allows him to commit seppaku, serving as his second to cut off his head after he had sliced his belly open (this was the first time I'd heard of this form of ritual suicide, so it made a big impression on me).
I don't remember any details about the cover or the author, but I do remember it as a hardback book, probably over a hundred pages. It might have still been marketed as a kid's book, but I was dipping into the adult section of the library at an early age, so I can't swear to that.