I know why Orochimaru is going to Tsunade.

OK no offense at all but do you know that Hashirama has senju DNA and do you know what Zetsu's are made of? SENJU DNA. 
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also Orochimaru was the one who injected Senju cells into Yamato. 
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So Orochimaru isn't going to tsunade for senju cells. 
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but the reason why is that he has been helping Sasuke and what does Sasuke want to do? be hokage! and how can he be assured of that? if tsunade dies!
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So Sasuke most have told Orochimaru ot kill Tsunade and that is what Orochimaru is going to do. Kill TSunade for sasuke to become hokage. 
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btw they are all weakned and have no hope since Sakura took their last hope away from them, so they will be a easy picking for Orochimaru.