Because an aspect of Gadianton crimes is murder for political gain, Nephi accuses his audience of accepting this evil along with the other manifestations of Gadianton wickedness. They are ripe for destruction because of their acceptance of the Gadiantons (v. 26), and this destruction is not in the future but right at their doors. The dramatic evidence Nephi offers is a just-committed murder, which also powerfully asserts his own prophetic calling. If he can accurately predict something so easily verifiable, then should not they believe that he can also predict the coming Messiah?
Text: This is the prophecy referred to in the superscription at the beginning of Helaman 7.