Regardless of what anyone says, Ziva knows for a fact that she didn't kill Brian Dempsey. If this had been a year ago, then she would have snapped his neck but given that she's spent a few months with NCIS, she's learnt a lot of humanity and empathy since then.
She has to hope that something else killed Dempsey.
Unfortunately, that's not going to be enough to convince Gibbs of her innocence. Ziva also knows that only Ducky's tests will help find out Brian Dempsey's real cause of death. She just hopes that it all comes back in her favour because she's not a killer.
But then again, only time will tell what really happened to Brian Dempsey.