Alma essentially said, “It is better that your one soul should perish, than that you should lead many people astray.” This is the same legal principle was invoked in the slaying of Laban. Alma was not saying, “We are going to kill you so that you will not mislead other people.” God was the one who had been offended by the blasphemy, so leaving the judgment to God and allowing him to curse Korihor with speechlessness made good sense and was perfectly fitting and appropriate.
The cursing of Korihor with speechlessness is interesting. His tongue had been the instrument of offense. There was nothing more fundamental to biblical jurisprudence than the idea that the punishment should suit the crime, that it should be tailored to match the wrong. So, Korihor’s tongue being cursed was a clear sign that what he had been doing with his tongue was inappropriate.
Book of Mormon Central, “Why Was Korihor Cursed with Speechlessness? (Alma 30:50),” KnoWhy 138 (July 7, 2016).