The Killing Way — Tony Hays

It is the time of Arthur, but this is not his storied epic. Arthur is a young and powerful warrior who some would say stands on the brink of legend. Britain's leaders have come to elect a new supreme king, and Arthur is favored. But when a young woman is brutally murdered and the blame is placed at Merlin's feet, Arthur's reputation is at stake and his enemies are poised to strike. Arthur turns to Malgwyn ap Cuneglas, a man whose knowledge of battle and keen insight into how the human mind works has helped Arthur come to the brink of kingship.

Malgwyn is also the man who hates Arthur most in the world.

After the death of Malgwyn's wife by Saxon hands, he became Mad Malgwyn, killer of Saxons and right-hand lieutenant to the warrior Arthur. Right hand, that is, until a Saxon cut his sword arm off and left him to die on the battlefield. Arthur rescued him. Now a one-armed scribe and a heavy drinker, Malgwyn rejects the half-life that his liege gave him. But loyalty is sometimes stronger than loathing...and Malgwyn is pulled toward a puzzle that he can't walk away from.