There's no real reason to pass on the demon king's reward, though.
Even if you don't want to kill anyone, you should take it and then just not kill the other heroes -- it'll give you the exact same gift as the goddess, and it doesn't mention a penalty for failing to kill the other heroes. It reduces the chances of it going to somebody who does want to kill the other heroes.
I guess the existence of it (since the top 10 heroes will see it) will guarantee everyone's paranoid and sets up the betrayal plotline, which I guess is the point.