Soh's reactions throughout the game is one who knows the oath and the ramifications of it.
But if we believe Soh's feelings for Yoshiro, her descendant, are what causes Cursed Soh to emerge, then moments like this are out into better perspective.
She's witnessing the Path from outside. This is what she endured and to see it happen to someone else affects her differently this time. Similar to how we tolerate things for ourselves but not our children or loved ones.
Soh's personality is entirely told through subtle body language and Yoshiro's reactions to her in the tent. They're obviously very close and she's no doubt watched over the girl since she was younger, as we've seen in the normal ending that the Messenger watches over the Maiden
When you fight Cursed Soh, what are you actually fighting?