Cats are lovely but can be so cruel. I know its just natural instinct, but...
You did a good thing, what a horrible experience it must have been. Poor duck must have got hit by a car. Its a bad time of year for them, saw at least two that have been hit on my way to work this morning. I even saw a live one wandering across the road, of course I slowed down for it.
I wonder if its because they are weighed down with eggs that they choose to walk rather than fly?