Oh no, I guess there's not much you can do about that. I know this isn't much consolation but you did the best you could. You can't make them not kill them. I suppose you could go kidnap them but that's not realistic is it?
I'm sorry you feel so bad, I would too. I don't let people buy my ducklings after hatch and I don't let them hatch my eggs unless I know 100% that they will be good to them. In fact, when I have too many drakes I will just deal with it instead of just "giving" them away. I want to know where they will live, are they protected and safe. I actually interview people before I let any of my ducks go to a new home.
I had experiences last year that people didn't take care of my babies or the eggs from my babies that they hatched and it was just awful so I won't do it anymore. I still feel terrible about it but I learned. You've just learned a lesson just like I did last year.