Alma 21:3

Now the Lamanites, of themselves, were sufficiently hardened, but the Amalekites, and the Amulonites, were still harder; therefore they did cause the Lamanites that they should harden their hearts, that they should wax stronger in wickedness, and their abominations.

The Lamanites were already stubborn. But the Amalekites and Amulonites were even more stubborn. They made the Lamanites become more stubborn too. They got stronger in doing bad things.

The Lamanites were already hardened enough on their own, but the Amalekites and Amulonites were even worse. They pushed the Lamanites to harden their hearts further and sink deeper into wickedness and evil practices.