Selecting good people to be protected from the angels of destruction and wicked people to be identified for them. But Justice asks: how different are they really?! The Gemara goes on to explain the rebuke in place here. The discussion as to how and why the the tzaddikim were marked at all emerges from why it was done with the Hebrew letter Tav. Also, Zekhut Avot, and what it means to not rely on it. Plus: the story of Reuben and Bilhah in the biblical text as prep for tomorrow's daf, which elaborates on those who never sinned, despite all appearances to the contrary (including Reuven).