In the vernacular of the Book of Mormon, to “destroy” did not mean annihilation. It meant to end the organized existence of people or to terminate their independent government, deprive them of a land, and end their cultural dominance. In the Book of Mormon, people were destroyed when they lost control over their government and land. Their ability to preserve their own values and choose the way they were governed was imposed on them by others. Often, but not always, it was from a different ethnic group. Once people were destroyed, they were oppressed and suffered. Often they were oppressed with grievous taxes and had religious liberties removed.
Destroyed people faced a choice: either repent, in which case they came through the period of oppression with another chance; or if they were angry and rebellious, they would then be “swept away.” Being destroyed is not at all the same as being “swept away.” It is possible for people to have been destroyed and not even realize it. But when they are “swept away,” they face extinction and cannot help but notice it.
Book of Mormon themes about destroying people are not history, but prophecy. They are intended to awaken people to things as they happen, so they will understand and repent before it is too late.
God has decreed there will be “a full end of all nations.” T&C 85:3. Despite this, His people have also been told, “Fear not, O Jacob my servant, says the Lord, for I am with you. For I will make a full end of all the nations to which I have driven you. But I will not make a full end of you[.]” Jeremiah 17:4.