OK but seriously, this is a super 4d chess move by Eris. Xivu Arath's motivation isn't just being a commander for the Witness, she is here to exact revenge on her sister for betraying the Deep, get revenge on us for killing her brother and leaving his throne empty, defying the sword logic. She even said so much in the Ghosts of the Deep fragments.
She is here for revenge, and her kills in vengeance could very well siphoned off by Eris, giving her power. If that's the case, we have her in an unwinnable situation. Her vengeful warpath is fuel for Eris, making her more powerful, if she stops, her worm consumes her, and with their vast conquests across the universe, there likely isn't another civilization outside the sol system that could satisfy her worm's hunger.
Don't forget the loophole that comes with Guardians in the picture: you can kill them forever and it still counts.
