Actually lycanthropes can live off of berries and other non-living food sources, it is unknown why they have such high desire to kill.
"Hircine, whose sphere is the hunt, the sports of Daedra, the greatest game, the chase and
sacrifice of mortals; known by the names The Huntsman of The Princes, and The Father of Manbeasts."
I would think that werewolves reflect Hircine's sphere. Hunting by night while being hunted by day. But there is more to that. In Bloodmoon and Daggerfall, a Lycanthrope had to kill a person or more in order to wake up healthy in the morning. In order to have the blessings of a werewolf during the bloodmoon in wolf form, the werewolf had to kill a person and sate their bloodlust. Proper sacrifice to Hircine. And smiling upon that sacrifice, Hircine grants them the blessing.
"The hunger or the Bloodlust of the Lycanthropes is legendary. Unlike Vampires that can survive without blood for a period of time, the Lycanthropes cannot. They
have to kill once they have turned to beast form otherwise they would die. Certain artifact can suppress this Bloodlust, but the artifact is very rare and difficult to retrieve."