The vehemence of this response might surprise a modern reader accustomed to assuming that the Nephites were good guys, and clearly in the right. For the many Lamanites, this was not the case. Their traditions had taught them to hate. Upon just a little reflection, we may understand that hatred between peoples has too long been a facet of our human nature. Many have hated on no better basis that this Lamanites assumptions about the Nephite motives.
The particular reasoning here is that the Nephites are a threat to Lamanite property. From what we know of the Nephite record, there is no justification for this assumption. However, from the Lamanite record, the theft of the birthright from Laman creates the tradition of the Nephites desiring and obtaining that which rightfully belonged to Lamanites.