Alma 20:16 - 20:20
Alma 20:16 Now when his father had heard these words, he was angry with him, and he drew his sword that he might smite him to the earth.
That is not what the father wanted to hear from his son and drew his sword to kill him.
Alma 20:17 But Ammon stood forth and said unto him: Behold, thou shalt not slay thy son; nevertheless, it were better that he should fall than thee, for behold, he has repented of his sins; but if thou shouldst fall at this time, in thine anger, thy soul could not be saved.
Ammon stood between Lamoni and his father saying that it would be better that Lamoni died instead of his father. Lamoni was prepared to meet his maker as opposed to his father who had not repented of his sins and would be unprepared to meet his maker.
Alma 20:18 And again, it is expedient that thou shouldst forbear; for if thou shouldst slay thy son, he being an innocent man, his blood would cry from the ground to the Lord his God, for vengeance to come upon thee; and perhaps thou wouldst lose thy soul.
For that reason alone, Ammon tells Lamoni's father to back off. If he should kill his son, justice would demand that his father pay the price of that slaying in the eternal world to come.
Alma 20:19 Now when Ammon had said these words unto him, he answered him, saying: I know that if I should slay my son, that I should shed innocent blood; for it is thou that hast sought to destroy him.
Lamoni's father acknowledges that would kill an innocent man if he should kill his son, but it is Ammon's fault that he has to take this action.
Not sure of the logic of Lamoni's fathers argument. Is his son's death to save him from Nephite corruption or to prevent him from becoming a Nephite sympathizer and therefore an enemy to the Lamanites. Any Nephite spies would have to be put to death.
Alma 20:20 And he stretched forth his hand to slay Ammon. But Ammon withstood his blows, and also smote his arm that he could not use it.
Lamoni's father then tried to kill his son, but Ammon defended him but hitting his arm which prevented him from using his sword.