Verses 6 and 7 continue the litany of losses. The combined forces against them are overwhelming, and the Nephites cannot resist. They are continually fleeing, and for those who could not flee, the result was in living under a conquering nation and destruction.
Verse 8 begins a transition into Mormon’s expression of his feelings about this final war. It is terrible, but he does not desire to dwell on the destruction. What he wants to do is use this story as a warning for the future, to declare that “all things which are hid must be revealed upon the house-tops.” The “things which are hid” are the sins of the Nephites in abandoning their God.
To expose these things on the house-tops means to write them in a message for the future. That begins in verse 9. Mormon is writing so that “a knowledge of these things must come unto the remnant of these people, and also unto the Gentiles.” He is writing “a small abridgment,” which is the Book of Mormon.