Stone (Gollancz Sf S.)
 

He's the universe's last criminal--set free with orders to destroy a planet's entire population. If he succeeds, that act would tear apart an interstellar utopia that has existed for centuries, where humans have their every need taken care of by nano-tech machines. To elude detection as the crime is being prepared, the murderer voyages to numerous worlds. In these exotic and varied landscapes, he tries to reawaken his killer instincts--all the while wondering who is behind the contract. In a society that has forgotten how to commit crime, who could possibly aspire to genocide?