The words "that he should die" are understood from the previous clause. Alternate translation: "and that he should die not only for the nation" (See: [[rc://en/ta/man/translate/figs-ellipsis]])
This is an ellipsis. The word "people" is implied by the context. Alternate translation: "would be gathered into one people" (See: [[rc://en/ta/man/translate/figs-explicit]])