Why dad ducks kill there babies?

Because they dont really know how to take care of their babies and to him they're food.

And they dont really kill their babies.

I improved your answer (grammatically and spelling-wise), but it's not quite right.

Ducks are pescitarians. It means that they are vegetarians, but they eat fish. A duck will not eat a cat, or another bird, why would it eat a duckling? Mother and father ducks both on occasion will kill their own offspring, mostly for a territorial reasons, and sometimes because they are in a low food area, and can't afford to have more competition for food. It is much more common for male ducks to kill their young, but not unheard of for mother ducks to as well.