She... "died". Then another enemy "appeared" that seems to know the project. It seems like that weird shinigami has some feelings, too, even if it's for his creation, and set self destruction on the hand.
Love those psychological bits Kubo interjects into his works. We already seen it with Ichigo's Zaraki hallucination, but this time it is even more fitting as a way to see into Mayuri's psyche.