"And again, when they thought upon the Lamanites, who were their brethren, of their sinful and polluted state, they were filled with pain and anguish for the welfare of their souls."
King Mosiah had just finished reading the records of Zeniff and the account of Alma and his followers after they fled from King Noah. The people of King Mosiah experienced mixed emotions over what they heard. This verse shows their sorrow over the Lamanites. Yes, the Lamanites are their brethren, but they are also their enemies. Yet, the Nephites were not merely sad about the Lamanites' sinfulness, they were "filled with pain and anguish." I know I'm not usually very bothered when I hear that bad things have happened to bad people, or even to someone I don't particularly like. Usually, at these times, I hear that inner voice that tells me I'm a better person and they deserve what they got.
These Nephites were true followers of Jesus; they had charity. Instead of allowing the Lamanites' wickedness to make them feel better about themselves and justify any actions they had to take in defending themselves against them, they worried over their souls, fearing for their eternal salvation.