Alma 30
The reason that the people of Ammon were not fooled by Korihor was that the doctrine he taught – that whatever a man did was no crime – was the same philosophy they had grown up with and they knew the light of the gospel was better than that lifestyle.
I also note that Korihor was never tried by a judge for he had committed no crime. He contended with Alma who exposed his lies, but his destruction was an act of God and his death came at the hands of people who did not believe in God nor have care for their fellow men.