12 Yea, I say unto you, that in the latter times the promises of the Lord have been extended to our brethren, the Lamanites; and notwithstanding the many afflictions which they shall have, and notwithstanding they shall be driven to and fro upon the face of the earth, and be hunted, and shall be smitten and scattered abroad, having no place for refuge, the Lord shall be merciful unto them.
13 And this is according to the prophecy, that they shall again be brought to the true knowledge, which is the knowledge of their Redeemer, and their great and true shepherd, and be numbered among his sheep.
Samuel chastizes the Nephites for despising the Lamanites despite their newfound righteousness. Tribalism and hatred had prevailed over Christlike love. Nevertheless, Samuel preaches an inevitably unpopular sermon that not only were the Lamanites more righteous than the Nephites, but that the Nephites would be destroyed while the Lamanites would eventually be restored and thrive. Is there any wonder that such an unpopular sermon would lead to violent backlash? 

Just like the Jews in Jerusalem, the Nephites had become assured of their righteousness. They saw themselves as chosen and special. Even though they were wicked, they saw themselves as the only ones who were truly good. That baleful pride meant that they would despise and even try to kill a prophet that would speak out against them.