Kyle Bain, a member of the Knights Temporal, goes on a mission to prevent a murder in the past because that’s what the Knights do—regardless of whether the murder may be just or not.
Thou shalt UN-kill, whenever possible.

Variants

(1)
  1. “Prevenge” by Mike Resnick and Kevin J. Anderson, in Analog, November 2006.
  2. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . written by Mike Resnick