Alma 53:15

"And Helaman feared lest by so doing they should lose their souls..."

Helaman feared that if the people of Ammon broke their covenant and fought the Lamanites they would lose their souls.  Helaman understood the importance of keeping covenants made with the Lord. It is better to suffer and die than to break sacred covenants. So many blessing come when covenants are kept and the opposite it true when they are broken.