I just completed the mission where you have to kill Crispin in order to
assume his identity for the auction.
Near the end of the mission, Clara says something along the lines of
"Maybe we had to kill Crispin, but at least we can save the girl" (Poppy)
I retried the mission many times attempting to beat it without killing Crispin, but I can't get close enough to him before he runs away.
So, is it possible to knock out Crispin, rather than killing him? Does it affect the storyline or the dialog at all?