While Sayid could've mistaken Kate to be evil incarnate, Sayid didn't stab Kate because Dogen referred to the person as a "him" and as someone who is dead. Last I checked, Kate isn't a "him" and Sayid had no reason to think Kate died, but did know that Bentham did.

Bravo Noel Murray!
Let's give this guy props! Yet again, Noel somehow combines his sharp insight and clever musings with perfect cross-references and subtle observations to create a splendid recap of the whole episode. Bravo indeed.