True to the secret combinations of the Jaredites, Kishkumen used a secret organization to attempt to kill Helaman in the same way that he had killed Pacumeni. Mormon does not tell us how a servant of Helaman had learned of, and infiltrated, the organization that would become known as the Gadianton Robbers. Nevertheless, Helaman has a servant who knows enough to know what they plan, and to know how to give a sign so that Kishkumen would believe that he was a co-conspirator.
The servant kills Kishkumen, and reports to Helaman. Helaman clearly knew enough of what was happening that there was no question about the legality of what the servant had done.