The Lamanites felt completely wronged and betrayed! The verses that are missing here, verses 13-16, talk about more reasons that they felt wronged, but I feel like verse 12 states it well enough: they felt that Nephi and Lehi had wronged Laman and Lemuel. Now, what is the consequence of this? Verse 17 shows us that because of this belief, they were taught to hate and murder the Nephites.
What we learn from this is the danger of perceived wrongs. Because of the wicked tradition of the fathers of the Lamanites, they perceived that they had been wronged. Even if this would've been real, they have held onto that hatred for so many generations that it has continue to grow inside of them. This principle is similar to the way we interact with other people. Are we actually be wronged or just perceiving it that way? If we are actually wronged, why haven't we forgiven them? Are we letting that grow into a hatred?
The Book of Mormon teaches us things TO do and things NOT to do, and this is one of the things that we learn not to do. Let us all forgive others and never perceive wrongs that aren't actually there.