The Gadianton judges have a vested interest in discrediting Nephi as a prophet. Nephi has prophesied against them directly, and if Nephi is honored as a prophet, then the people might remove them from power on Nephi’s word alone, becoming a self-fulfilling prophecy. They certainly don’t want to take that chance, and so rather than see Nephi’s prediction as prophecy, they declare it to be complicity. Nephi knew, not because he was a prophet, but because he was involved in the nefarious deed.