Ducks are carnivores. While they eat plants too. They love bugs, fish, worms, frogs, and any other small things they can grab. Their bills are serrated. So even though they have no actual teeth, those bills can hold prey. And in the ccase of fish they can turn a fish around in their bills so it goes down head first.
Also, ducks are mean. They will attack each other if new ducks arrive at a pond. When one duck wants to dominate another, they will grab a wing shoulder and try to dunk the other duck, holdings head under water. Many times they will gang up on a new duck, attacking him, even killing him if he can’t get away.
This is normal mallard behavior especially in the fall amongst unattached ducks gathering to fly south.
Watch out, they are about to evolve back into T-Rex. All bets are off then.