Alma 37
I don’t remember if I noticed this in previous readings or not, but Helaman was given the same charge as Joseph Smith regarding the records, that if he transgressed they would be taken from him and he would be delivered to the buffetings of Satan but that if he were faithful no power of the adversary could take the records from him. Alma told this to Helaman by the spirit of prophecy.
The Jaredites were not destroyed because of their bloodthirstiness, they were destroyed because of their secret oaths to kill all righteousness. They had covenanted against God and vowed to destroy any prophet he might send among them that they might pursue their wickedness without opposition. Had they simply been savage and bloodthirsty without the oaths, as the Lamanites were after the Nephites were destroyed, they might have been allowed to survive as a people rather than be cut down to the last man. Their vow was to corrupt or kill all whom they came in contact with.