Alma 44:17

And now Moroni was angry, because of the stubbornness of the Lamanites; therefore he commanded his people that they should fall upon them and slay them. And it came to pass that they began to slay them; yea, and the Lamanites did contend with their swords and their mights.

Moroni was angry because the Lamanites were stubborn. So he told his people to attack them and kill them. They started to kill them. The Lamanites fought with their swords and their strength.

Moroni, angry at their stubbornness, ordered his men to attack and kill them. The slaughter began. The Lamanites fought back fiercely with everything they had.