Because of their murders

1. Alma 18: 2
     2 And when they had all testified to the things which they had seen, and he had learned of the faithfulness of Ammon in preserving his flocks, and also of his great power in contending against those who sought to slay him, he was astonished exceedingly, and said: Surely, this is more than a man. Behold, is not this the Great Spirit who doth send such great punishments upon this people, because of their murders?
2. 3 Ne. 9: 9
     9 And behold, that great city Jacobugath, which was inhabited by the people of king Jacob, have I caused to be burned with fire because of their sins and their wickedness, which was above all the wickedness of the whole earth, because of their secret murders and combinations; for it was they that did destroy the peace of my people and the government of the land; therefore I did cause them to be burned, to destroy them from before my face, that the blood of the prophets and the saints should not come up unto me any more against them.