What made Tim O'Brien throw the grenade that killed the young man on the trail?

O'Brien killed the man because he was afraid.... he threw the grenade without thought. It was instinctual.

I was afraid of him—afraid of something—and as he passed me on the trail I threw a grenade that exploded at his feet and killed him.

It was entirely automatic. I did not hate the young man; I did not see him as the enemy; I did not ponder issues of morality or politics or military duty. I crouched and kept my head low. I tried to swallow whatever was rising from my stomach, which tasted like lemonade, something fruity and sour. I was terrified. There were no thoughts about killing. The grenade was to make him go away—just evaporate—and I leaned back and felt my mind go empty and then felt it fill up again. I had already thrown the grenade before telling myself to throw it.