To see his Sif cry broke Thor's heart.
He had seen her cry only twice before in her life. Once when Heimdall was injured, on the cusp of death, and Sif sat by his bedside, clutching one hand in hers as tears silently stained her cheeks.
The other when they were children, and her tears had fallen in frustration at the insistence of their teachers that she learn to weave and embroider while Thor and Loki learned to fight.
But today she wept. Bitterly, brokenly, and he could do nothing to soothe her.
He had been the first in the room when he heard her cry. He had been the one to find her, hair shorn close to her head and clippings spread like a halo on her pillow.
That was a week ago.
She had been stunned for several minutes, her hands dripping with gold as she looked at him in askance. Thor could say nothing. Finally, after a lifetime of silence, she met his eyes.
"Your brother did this," Sif said.
Thor sighed. "I know."
Sif extracted her revenge in the way of the Lady Sif- at swordpoint. It was only Thor's intervention, and later Frigga's, that caused her to spare Loki's life, and Odin's requirement that he fix his misdeed and make amends.
Loki presented her with a headpiece, magically enchanted, he claimed, to be more golden than the sun, to grow longer and fuller with every life she extinguished. Thor watched, his heart swelling, as Sif smiled, though it was wary, and allowed the headpiece to be fitted.
He watched as Loki made an apology. Though it was made at knifepoint, it was still an apology.
He watched with hope as his brother and his love made a kind of grudging peace, as they agreed, in what was their way, that they would do their best to coexist.
And he watched the next morning, as Sif tore at the now onyx locks, as she screamed and raged and swore Loki's death at her hand.
He watched as she fell onto the bed, exhausted and heartbroken, with the strands of unbreakable, immutable raven hair covering her face like a shroud.
"I am his," she whispered. "He has marked me."
Thor shook his head and gathered her into his arms. "You are yours," he said, pushing her hair back to see her face, "despite my brother's wrongs."
A tear tracked down her cheek, and he kissed it away.
"I will kill him," she whispered.
Thor said nothing. She would not kill him, because Loki would be sent from Asgard as soon as the Lady Sif had exhausted herself in rage; he would be kept at a distance until it was safe for him to return. If it ever was.
Perhaps it was unfair of him to guard his brother in the face of this wrong. Perhaps he would be better off to let Loki face Sif and reap his reward.
His heart broke at the thought of it.
Sif would not soon forgive him, that he knew, but if he allowed her to kill Loki, he would never forgive her.
It wasn't fair, and it wasn't right. But it was the way of the two sides of his heart, and Thor would die for them both.