So Negi decided for her to use the nuclear option because she felt cornered or left behind. By demonizing her I can only a few options to continue:
1) Immediate solution of the tension because Fuuts realises that she is not Miku (very unlikely)
2) An arc like seven goodbyes where she redeems herself.
3) Pushing this trope further by making Miku give him a birthday present that matches with the "I'll be supporting you" theme and increase the tension.
My money is on option three.