Perhaps it was not her duckling or the duckling was ill. Sometimes they kill the smallest one to give a chance to the bigger ones.
I also think that the baby may have been sick. I think that a lot of animals in nature kill babies if they are sick or weak.
All barnyard animal's are capable of this. One of my oldest best cochin hen's killed a baby polish chick that got thru behind some wire away from it's mother.Turkeys and geese are the worst for killing baby duck's, chicks etc.It's sad but it happen's.
,may be becus it was being a bad duck meow meow meow lol lol lol may be the person wanted to maybe the person lol hated them maybe it was bugging meow meow meow lol lol lol..