Why didn't she kill him?
Why didn't Mercedes kill Captain Vidal, when she had the chance?
shareFor normal human beings, it isn't an easy thing to take another person's life, no matter the circumstances. Mercedes wasn't a killer.
It might be worth noting that her name means Mercy.
_____
Strip away the phony tinsel of Hollywood and you find the real tinsel underneath.
But they do kill him later same night. Not her personally, but she sure did not show any mercy then.
shareAt that point, he had murdered Ofelia. Before that time, he had killed men who were soldiers or their sympathizers or people he had mistaken for their sympathizers. Maybe Mercedes had run out of mercy for him after he murdered an innocent young girl who had actually been his stepdaughter.
_____
Strip away the phony tinsel of Hollywood and you find the real tinsel underneath.
That raises two questions:
How about the two innocent farmers that he killed? Surely most viewers of this movie lost any sympathy they might have had for him by then, so shouldn't one think that Mercedes did too? When she got the rabbits, you could see by her face that she knew that he had done something terrible to get them.
And did she know that he had killed Ofelia? We see Ofelia die not only in slow motion, but also quite slowly (depicted as she enters the realm of the Faun), but Mercedes doesn't see that she had been mortally shot.
If she figured out that Vidal had killed Ofelia without seeing it, surely one should expect that she could figure out what he had done to those poor farmers, hence losing all sympathy.
If she didn't kill him for mercy's sake, I doubt that it had much to do with feeling sympathy for Vidal as an individual. It probably had more to do with an instinctive horror of taking any human life. It probably had less to do with how she felt about Vidal and more to do with what kind of person she saw herself as being - namely, not someone who usurped vengeance or justice but who left them in God's hands.
Edited to add: And even after he killed Ofelia, Mercedes didn't try to kill him herself; she simply didn't try to save him from the rebels. Which probably would have been fruitless anyhow.
_____
Strip away the phony tinsel of Hollywood and you find the real tinsel underneath.