Violent portrayal about how terrible violence is Hacksaw Ridge

In a way, it's a contradiction

When he was a young man in West Virginia, Desmond Doss (Andrew Garfield) made a vow to God that he would never touch a gun or intentionally hurt another human being. In "Hacksaw Ridge" there are two incidents that inspire that vow-one, when he is a young boy and hits his brother with a rock. For a few terrible moments, he thinks he's killed him. Then, when he is a teenager, he takes a gun away from his drunken, abusive, self-loathing father (Hugo Weaving) and almost uses it against him. Which of these is the moment he makes his vow? In my life, it's usually not one event but a collected series...