One of the important aspects of the Gadianton catalogue is murder for political gain. Nephi tells his audience that their acceptance of the Gadianton program is also accepting the ills of the Gadiantons, including this most vile murder for gain. In verse 26 Nephi tells them that they are ripe for destruction because of their acceptance of the Gadiantons, and now he tells them that this coming destruction is right at their doors. The evidence is that one of these murders for political gain has just taken place.
Nephi dramatically proves not only the corruption of the Gadiantons, but his own prophetic calling. He gives the people a prophecy that can easily be tested. If Nephi can accurately predict something so obviously come true, then should not they believe that he can also predict the coming Atoning Messiah?
Textual: This is the prophecy referred to in the header of this original chapter, now found at the beginning of chapter 7.