All My Sins Remembered — Joe Haldeman

NO CRERTURE IN THE GALAXY IS DEADLIER THAN MAN.

Otto McGavin, one of twelve Prime Operators in the universe, worked for the clandestine arm of the trans-space peacekeeping agency as a ruthless guardian of alien rights. He traveled from planet to planet--his body encased in plastiflesh, his mind disguised by personality overlay--infiltrating bizarre alien cultures, surviving by raw instinct and violent assassination on exotic, bloodswept worlds.

And always he returned to his original self--his conscience stabbed by the memory of all those he'd killed in the service of interstellar harmony.