Why the NPCs No Longer Fish in the River

I know that DMs telling about their sessions are the second most tedious species alive (better only than players telling about their characters) but please do forgive me this indulgence.

The Place: Fantasy version of Russia, winter.
The PCs want to cross a river to get to town, but the local rusalka won't let them. She's been wronged by men her all life and now won't let any man cross the river in which she drowned. The PCs, being travelers from a distant land, are eager to prove to her that not all people suck and ask her for a quest to prove their worth. She tells them that she was kicked out of her home in winter along with her baby daughter and wants nothing more than to be reunited with her child - something that can only be accomplished if the PCs get her the bones of the dead child.

Are you positive this is your villain?

The PCs go to the death site and find a gnoll camp there. The leader of the gnolls is a fierce werewolf wielding a flail that can fell trees. They charge screaming bloody mayhem. There is a bloody battle and the PCs eventually defeat the gnolls and their werewolf leader and start looking for the old bones that will reconcile the rusalka. Meanwhile, the dead werewolf returns to its human form - a young woman remarkably similar to the rusalka.

Gnolls eating dinner, an activity deserving termination with extreme prejudice

Questioning a wounded gnoll, the PCs discover that the gnolls found a dead woman and a living baby. Believing the baby had killed its mother, a great honor in their culture, they adopted it and raised it to be the scourge of their canine deity, who in turn blessed the child with the gift of lycanthropy. At this point, the truth downs on the PCs.
Not even a bit crestfallen, and lacking any semblance of tact, they come to the rusalka and tell her "eh, we found your daughter, but we had to kill her, because she was a werewolf... sorry about that." They then dump the corpse of the daughter into the river, thus technically completing their quest.
At this point the rusalka wails about how incredibly cruel they are; to give her hope, just to dash it. Frothing and raging, she swears that she won't let anyone ever cross the river again.