1) He was about to act as a key witness in an important murder trial. Somebody had him silenced.
2) He unfortunately has the same name as somebody powerful, who an out-of-town assassin was hired to kill. It's a case of mistaken identity.
3) Superstitious builders have sacrificed him according to ancient rites so that a structure they are erecting will stand, after the last two attempts have crumbled to the foundations.
4) He's the illegitimate child of a local cleric who is supposed to be celibate on pain of death, and who he has been blackmailing. The cleric has killed him in desperation, to keep his secret.
5) He was a randomly selected victim, whose death was intended to demonstrate the wonderful efficiency of a new brand of poison, recently brewed up by a practicing alchemist. A local rogue lord has offered to pay the alchemist a goodly sum for exclusive use of the stuff, but wanted to be sure of its effects first.