Okay, doing that to her was badass, but why would he "help" her and not kill her? He just killed countless goblins that just followed her orders but he spares the person that ordered to kill Biscuit in the first place? A sadistic villain might do that to his enemy to see him in agony, but the orc with the highest wisdom of the realm?

The only reason to spare her that I can come up with is that she is still needed for ome prophecies to fulfill. She might have an end like Gollum.