As was said, Kabuto chose Orochimaru and has had the opportunity to leave him or interfere with his plans.
"As a young boy, Kabuto was found amongst the dead enemies after the Battle of Kikyo Pass by a chief medical officer of the Konoha Medical Unit Squad. The officer brought Kabuto with him back to Konoha, raised him as a son, and taught him medical jutsu when he got older. At some point in time, Kabuto was recruited by Sasori of Akatsuki, who sent Kabuto to spy on Orochimaru by using Brainwashing Investigation Technique to transform him into a sleeper agent. Orochimaru discovered this, and released Sasori's control over Kabuto's memories, leaving Kabuto with the option to follow him. With a little persuasion, Kabuto agreed to side with Orochimaru."
This is for anyone who wants to know his background history.
Kabuto is indeed a strange character.