Alma 37:26 - 37:30
Alma 37: 26 And now, my son, we see that they did not repent; therefore they have been destroyed, and thus far the word of God has been fulfilled; yea, their secret abominations have been brought out of darkness and made known unto us.
However, it is apparent from the record that the people did not repent and were destroyed. So having a record of their abominations and destruction is fulfillment of the prophecy that these evil acts would be revealed to the world.
Alma 37: 27 And now, my son, I command you that ye retain all their oaths, and their covenants, and their agreements in their secret abominations; yea, and all their signs and their wonders ye shall keep from this people, that they know them not, lest peradventure they should fall into darkness also and be destroyed.
Alma tells Helaman, he can reveal the history on the plates but he is to keep the secret oaths and covenants that these people made among each other a secret. Alma is afraid that if this knowledge became public that the people would adopt them and they should become like the Jaredites.
Alma 37: 28 For behold, there is a curse upon all this land, that destruction shall come upon all those workers of darkness, according to the power of God, when they are fully ripe; therefore I desire that this people might not be destroyed.
The reason being that there is a curse on the land by the Lord that if anyone practices these evil practices, that they will be destroyed as well.
Alma 37: 29 Therefore ye shall keep these secret plans of their oaths and their covenants from this people, and only their wickedness and their murders and their abominations shall ye make known unto them; and ye shall teach them to abhor such wickedness and abominations and murders; and ye shall also teach them that these people were destroyed on account of their wickedness and abominations and their murders.
So Alma commands Helaman to keep these secrets from the people but he should publish their history. The history will help the people to know what can destroyed them if they practice such things.
Alma 37: 30 For behold, they murdered all the prophets of the Lord who came among them to declare unto them concerning their iniquities; and the blood of those whom they murdered did cry unto the Lord their God for vengeance upon those who were their murderers; and thus the judgments of God did come upon these workers of darkness and secret combinations.
One of the things the plates reveal is that the Jaredites killed any and all prophets that came among them to call the people to repentance. And these martyred prophets, from the grave, called upon God to avenge their deaths upon the people who murdered them.