I had not thought death had undone so many.
In Dante Canto when he says, "Could not believe, death had undone so many", Eliot directly connects this idea to his line saying, "I had not thought death has undone so many." The use of the word "undone" shifts between these two lines. In Dante, it is used as a shock as so many people have died. But for Eliot the people are not literally dead; it is more that something has "undone" them, so death is more a metaphor for numbness.