Textual: Mormon is explicit that he is presenting a moral to the story here. When he says “thus we see…” he is telling us that we should pay attention to the main point. In this case, his point is that “the Lord worketh in many ways to the salvation of his people.” This conclusion comes after the indication of the larger number of people converting to the gospel than those who died. Regardless of the historical issue of sources and numbers, it is clear that Mormon sees a particular moral in this story, and includes it specifically for this reason.
Literary: There is enough of a contrasting structure here to suggest that at one time it was stronger than it currently appears. The first two phrases are meant to be a contrasting pair:
…not a wicked man slain
…but…
…more than a thousand brought to the knowledge of the truth.
There is a contrast between the concept of slain and “brought to a knowledge of the truth.” The contrast could be better understood when we understand that a knowledge of the truth leads to eternal life.
The negative construction “not a wicked man slain” begs to be paired with “a righteous man given life.” Although the construction is weak in English, it is possible that the plate text was a tighter parallel that was not completely translated.