It's getting weaker. It's not going to live through this, probably, and if it did it might kill someone less lucky than her.
And in the world where this was all somehow- some sort of misunderstanding. And she's watching a very sick person die in front of her. It would probably be, on average, in most worlds, crueller to let them die here, slowly and in pain, rather than quickly. She thinks. She could pick it up and run to the nearest town and hope they somehow have healing magic, but it would keep trying to eat her and for all she knows it's got some terrible communicable disease and she's looking at a strangely shaped fast zombie.
And. It really does look like the situation is just that a maneating monster with a voice that sounds human just tried to kill and eat her and she's in very dangerous territory and she's wasting time agonizing over whether to put it out of its misery.
She doesn't know how to heal, or if that's even a thing in this universe for people who aren't her true loves, and she-
already knows what the right answer is, here, for what she should do next.