Nephite law proscribed executions without the approval of the governor. Yet, where there is a wicked will, there is a way. The wicked judges had to figure out a way to kill these prophets because they did not have the power themselves to order the executions.
Similarly, the chief priests and elders, when faced with Jesus of Nazareth, did not have the power themselves to legally consign him to death. Therefore, they needed the help of Pilate who knew that for envy they had delivered him (Matt 27:18). They also needed the help of the people, so they persuaded the multitude that they should ask Barabbas, and destroy Jesus (Matt 27:20). As the Nephites, who did not sin ignorantly, so the chief priests and elders did not sin ignorantly, for they were the keepers of the law. Yet they chose to kill the lawgiver.