In Chapter 19 of ACOK, Arya
finds Yoren's body in the aftermath of their battle with Ser Amory Lorch and his men.
And she observes how he died:
"The axe blow that had killed him had split his skull apart"
But who killed him?
Science Fiction & Fantasy Stack Exchange is a question and answer site for science fiction and fantasy enthusiasts. It only takes a minute to sign up.Sign up to join this community
In Chapter 14 of ACOK, Arya
Throws an axe into the wagon for Jaquen, Biter, and Rorge. Rorge catches it and starts hacking at the wagon.
In the next chapter, they do not find an axe with the Night Watchman's body.
Later on in AFFC Chapter 37, the "hound" is killed:
His axe fell from limp fingers, and the two of them slammed together, Brienne's face mashed up against the dog's head helm.
Notice how this character wielded an axe, and that he is described as "thick and powerful" in this summary:
Rorge is described as squat, bulky, and hairy, with black hair covering most of his body. He is also described as thick and powerful.
And consider him and his companions:
Rorge is a strong man with nothing left to lose (either join the Night's Watch or die), who interestingly joins the Lannister guard after Yoren's death. Biter is too feral and too wild to weild any weapon when fighting Brienne, much less an axe. Jaqen is a faceless man and should not go around killing people for no reason.
There is plenty of motive, evidence, and opportunity to infer this person killed Yoren.