Guilt
It comes, like a thunderbolt, from nowhere.
He's waiting in silence broken only by cricketsong and grass ripping against a horse's teeth, waiting because he's been ordered to do so, not because he understands why. And all in a moment the earth heaves and groans underfoot, and with a deafening roar the seeming-immovable stone walls before him rend themselves like an old garment. A crackle of lightning shatters the clouds and the towers writhe in their death throes, battering the outraged sky.
The horse rears, screaming, straining at her rope, and he nearly releases her, nearly lets her bear him away in mutual terror because the castle is coming down - dear gods, it's actually coming down - but one thought holds him, frozen with horror, every obligation of nobility and decency condemning him in a single appalling realization.
He should never have let that girl go back inside.