As you can see, Balthazar has managed to make Nouru really, truly angry. And that kind of thing never goes well for anyone - not even a god.
It wasn’t the fact that he’d killed her. That is a thing that happens in war. It was that he had used her as bait, hurt Aurene, and - however inadvertently, however temporarily - robbed Nouru of her identity, of her self. The last one was what stamped out any remaining mercy that she may have still found in her heart for Balthazar.