Why does Walowick fire at a steel drum?

Walowick fired at the steel drum because he was afraid. The men had just seen all of the dead villagers, and for a moment..... he loses control.

“There was a burst of fire behind me. I turned. Walowick was firing toward a steel drum that lay on its side… I reached for a grenade just as Simpson ran past me. He grabbed Walowick and threw both arms around him. ‘Easy, man! Easy!’... I watched Simpson holding Walowick.”