[He wants to tell John in return what he saw, but it's still too raw and too frightening. Maybe he's more of a coward than he thought he was, a disappointing revelation. As ever, stress and fear make him crueller and sharper than he would be normally.]

How? You had no idea it was Moriarty who spoke to you before, you clearly can't tell the difference between us, so how would you know it was him to help stop him? I can't risk that, John.