Kimimaro was the sole survivor of the Kaguya Clan, who were battle-loving barbarians. Being a rare owner of the kekkei genkai, Shikotsumyaku, Kimimaro's father, fearing his son's abilities, locked him up in a cage for most of his life, releasing him only when he was needed to fight for the clan. The Kaguya clan was wiped out when they attacked Kirigakure, seemingly for no reason other than desire to kill. Having underestimated their opponent's defenses, they were slaughtered, with only Kimimaro surviving.
With nowhere to go, Kimimaro began to wander, he was eventually found by Orochimaru, and recruited as one of his most loyal ninja. While belonging to a clan that loved killing, murder, and slaughter, and being the most powerful of this clan, Kimimaro was not truly violent, because he never really knew the outside world. Thus, the only reason he fought was because his clan needed him to do so, and being needed by someone was the one thing he truly desired. However, when he met Orochimaru, his desires changed to serve him, and to ensure the continuation of his master's dreams and ambitions. Kimimaro was selected to be one of Orochimaru's next bodies, primarily because of the rare kekkei genkai he possessed. However, Kimimaro was unfortunately afflicted by an unknown disease, and hence, was no longer useful to Orochimaru.