Board Thread:Theories and Speculation/@comment-1783334-20150507053750/@comment-25478058-20150511080102

I think it's Kabuto, because Kabuto was changed by Itachi when he used Izanami on him. I think he may have become obsessed with Itachi. So while Kabuto helped Sasuke during the war he may have since changed his mind because, at the time he helped Sasuke, Sasuke was against Konoha, but now Sasuke is apparently working for Konoha, the government that hurt Itachi so badly. So Kabuto could be against Sasuke for the same reason Sasuke himself wanted to end Konoha.

Also, Kabuto is a good fit because he is super strong (snake sage) and is involved in Orochimaru's experiments AND he has access to Obito's old hideouts full of Sharingan and Zetsu.