"Because I am about balance, and a little mercy here saves more in the long run. And because that goddess' sister asked me to get her sister back, not kill you. Killing you was implied, but not part of the mission request, leaving me to decide what happens to you. Of course, if you would rather we could do that whole duel to the death, and then I would be obligated to destroy the new dragon subspecies in the next room, instead of studying them as they naturally develop as I would prefer."