There's a bigger picture, Itadori is spy, as spy is Itadori After getting backstabbed, over and over, and over he decided that that was enough. He backstabbed him, like Itadori Had said, "You had backstabbed me over and over again, now I backstab you." As the black flasher walked toward the French stabber, He had also stated,"When you respawn, I'll kill you. If your Medic heals you, I will kill you and your Medic" As Spy tried to run away and hide, He had absolutely destroyed his ankle. The flasher had started getting closer and closer.
