Daud's entire speech to Corvo is this if Corvo has done a perfect Pacifist Run to this point, with Daud reflecting on all the things he himself has done — murdering for coin, with no care for who he was paid to kill — and the path that Corvo chose. That Corvo, too, was asked to kill — and always found another way, even after all he suffered at the hands of the people he was sent to murder. The absolute Crowner comes if Corvo sheathes his blade and steps back, silently granting Daud his life. Daud sounds almost awed, to the point that it's nearly a Tear Jerker.
Daud: And you choose mercy. Extraordinary.
- And it becomes even more heartwarming when he tethers away when you spare him. it's then that you realise that he could have escaped at any time, but chose to stay and either die or be forgiven.