In a Dark Wood — Michael Cadnum

A desperate sheriff in Sherwood Forest chases a bandit called Robin Hood

The boar charges and Geoffrey, the sheriff of Nottingham, stands tall with an iron spear gripped in his hands, waiting for the moment to strike. Just before the beast is upon him, Geoffrey stabs, catching it right between the eyes. After a bloody struggle, the animal's life drains out. The sheriff has mastered the hunt.
 
For his entire life, Geoffrey has served the king. He has worked for him, tortured others on his behalf, and killed at his orders -- and now he will be called to do so again. There is a bandit in Sherwood Forest, a marksman the townspeople call Robin Hood, and the king demands the sheriff bring him to justice. But the outlaw will not be captured easily, and tracking him down will force Geoffrey to commit unimaginable sins -- all in the name of the king.