I think Kabuto either wants vengence or knowledge. Someone who wants to observes, seeing how he has a lot of patients(pun ++) While pushing the envelope just like Orochimaru, he probably doesnt see revenge that important though.

That is what we can tell at the moment.

If he really is anyone other then Kabuto, then the possiblities expands tremendously. Being the chaotic version preferred because its fun to read. I am ready to be WOWWED