I'm a big fan of Arthurian historical fiction; not so much of murder mysteries. But in either genre, _The Killing Way_ seemed to me to fall short of the mark.
On the Arthurian historical novel side, its main weakness was an apparent lack of preparation by the author. Great historical or fantasy novels (e.g. Tolkein) make the reader feel that there is a whole world out there, as real as this one, even if the author is just giving us a tiny fraction of its story. _The Killing Way_ way left me with the impression that the author's world did not extend beyond the novel. This is illustrated (pun intended) most obviously by the map at the front of the book. It shows just a few of the southern Civitates of Britannia (where the action takes place), and a vast expanse of space from the Thames to Hadrian's wall (which is wrongly located), labeled simply "Saxon-occupied lands".
Hays has not even bothered to stick with the real geography of his very limited setting, near South Cadbury in Somerset --- He invests the gentle river Cam with deadly waterfalls and 50-foot cliffs for banks. There are also serious historical "howlers" e.g. having Picts from Ireland.
In terms of a murder mystery, I found the plot not very interesting. It was obvious from the start that the murderer was going to be one of Arthur's political opponents, and for me there was not much reason to care whether it was this one or that one. Towards the end of the book, it turned into a "boys own adventure". The protagonist and narrator, Malgwyn, having gone with almost no sleep for two days, "dizzy and exhausted" from having been chased up and down the countryside, manages to kill or disable in no long time three enemies: (i) by "heaving a spear ... [which] struck him in mid-chest"; (ii) "I hurled [a sword] ... [which] caught [him] in the stomach", and (iii) "I flipped my dagger over and threw it .... [it] caught [him] in the shoulder". Not bad for a one-armed alcoholic ex-warrior with a desk-job.
For a historical novel set in Arthurian Britain with real suspense and mystery, I recommend the "Albion" trilogy by Patrick McCormack starting with
The Last Companion: a Novel of Arthurian Britain.
For a "conventional" spy/action/crime/mystery novel steeped in Arthurian history, I recommend
Our Man in Camelot.