Alma is not an angel. He is aware of his human frailties, and one of them is wishing that he could do more than what he is able to do. Although he is contemplating the results of the sadness that resulted from the loss of life in the war, and the loss of the opportunity to repent (at least in this life), Alma also understands that God requires no more of him than that he is able to do.
Our modern understanding of the ability to continue to learn and repent after this life does not appear to have been part of Alma’s understanding. Alma appears to have understood God’s laws as applicable to mortality, and having some finality with the end of mortality. That is what he is saying when he says that “I know that he granteth unto men according to their desire, whether it be unto death or unto life.” There are two choices, eternal life in God or eternity without god, or spiritual death.
Alma understands and asserts our essential agency. God’s decrees are unalterable, but our responses to them depend upon our own wills. Our choices will determine whether we are ultimately saved or destroyed. Alma understands, and writes, in terms of opposites: black or white; salvation or destruction; and spiritual life or spiritual death.