Korihor asks Alma to plead with God for the removal of the curse. Korihor now clearly believes in God, and believes in Alma’s ability to communicate with God. Alma’s response is that since Korihor might continue preaching, Alma will not implore God, but will leave Korihor in God’s hands. This might seem a little harsh, considering the fate that befalls Korihor. Why cannot Alma forgive him now that Korihor has confessed?
Ultimately we do not know. Alma tells us only that Korihor might return to preaching against God. Since Korihor had secretly believed, but had been led astray, Alma has reason to believe that he was susceptible to the influences of the Anti-Christ position, and that he might return. Ultimately it was God who had laid the curse on Korihor, so Alma felt justified in leaving it to God to remove it.