Speak No Evil

Of the six billion people on planet Earth, only one is allowed to roam the stars and to taste of the delights of interstellar civilizations.
The one man who's allowed to roam the stars is a hit man, a contract killer, a very skilled hit man.
The one constant among all civilizations, and particularly advanced civilizations, is the prohibition of the taking of human life. The hit man lacks that prohibition.
He's sent on a mission to rescue a King.
The King is the key to eventually moving an alien planet to advanced civilization status.
There's action and adventure and a painful lesson learned.