It's hard to imagine that even Danzo was once so egoistic. (to a point, he still is).

The logical conclusion to why Sasuke opened his left M.Sharingan on Danzo as he skewered him was to use it (Genjutsu on Danzo to make him think it killed Karin and thereby letting her go).

But then... why the lightning blade in the preview that can only be for ending Karin's suffering?