At this point Samuel is about ready to wind up his discourse. Note that in spite of the rough and brutal way Samuel has been treated, he still refers to his persecutors as "beloved brethren." But he wants them to know what will happen if they reject his message.
The coming devastation will be particularly tragic for the expectant mothers and those with small children.
Samuel reminds the Nephites that they are a chosen people and God only chastens them when they are guilty of serious iniquities against the Lord.