Here’s an outstanding description from a document on the Devi Mahatmyam.
The one killed, is Shumbha. Why? Who is he? Or rather, what does his image depict?
Shumbha denotes the ego, the false sense of self, the limited self. The self (atman) is not the same as the non-self (anatman). It is ignorance that gives rise to the mistaken identity that the non-self is identical to the self. Devi kills the asura Shumbha (ego) by piercing with her spear of knowledge (jnaana). The death of the limited ego leads to the relinquishing of a limited identity, and gaining the realization of the true self.