I personally believe that he has split-personalities. The usual Kabuto is calm, soft, obediant, intelligent, sly, observant, and strong.

What we saw there way back when was something completely out of character for him. Never before and never since has he acted in such a way. I believe under the surface there lies the psyche of an extremely well trained maniacal killer. Which would be the complete opposing nature of his usual healer/obediant persona.

The best analogy I could give is that I think he's a Dr. Jekyll/Mr. Hyde.

But...what do I know, maybe its nothing.