The accusers suggest that Nephi might not have actually committed the murder, but that he was “confederate,” or a conspirator, with the person who did. They offer him the opportunity to escape execution and to receive money for his confession. There is no way to know if they were sincere in the offer. It would not have mattered. If Nephi accepted, he would be publicly discredited, which would accomplish their goals of removing him as a threat.
Nephi understood both that he was innocent, and that they, in their own way, were not. They were guilty of corrupt government, and Nephi saw his public trial as an opportunity to begin to reverse the hold they had upon the people. Nephi could show that they were corrupt, and that Nephi was a true prophet of Jehovah. That could lead many people back to their traditional Nephite religion.