I need help on identifying what kind of predator killed my duck.
I'm sorry I can't help identify what has gotten your ducks, but you need to have them in a shelter at night. Now that a predator has found an easy dinner at night, he will come back until they are all gone. Do you have something you can set up temporarily until a permanent structure can be built? I know not all duck owners think they should pen their ducks up at night - but this is why. A 6' fence just isn't enough protection for a determined predator. Can you put them in the garage tonight - or a shed? I am just afraid you will lose one a night, or every other night until they are all gone.
Where I live, there aren't a lot of daytime predators, but we have racoons at night. I get my ducks in their cozy little house every night at dusk, and I can sleep soundly knowing they are safe.
I hope the rest of your ducks are ok! It can be hard on them to see a friend hurt.
I agree with @jducour that you need to provide a nighttime shelter.