The collective trauma of war refers to how we all suffer simultaneously in wartime. It is empathy, understanding others’ points of view, at the extreme. War affects everyone, and both you and your enemy have pain. Consider this line from your quote:
He became simultaneously the stabber and the stabbed.
This epitomizes both the fog of war, and the collective trauma. He is both the victim and the perpetrator. He realizes that everyone is connected in war, and a heart against one hurts us all.
He goes on to say:
He could feel both the impact of the bayonet as it entered his victim’s body and the pain of having his internal organs slashed to bits."
He could feel his blade as it entered his victim. He is empathizing with his victim. He is a solider, killing so he won’t be killed, but he does not want to cause harm to others.
In the short story “Guests of a Nation” we have a similar theme. Both the rebel Irishmen and their prisoners, English soldiers, are involved in the collective trauma. They live together in close quarters, and might have got along if they had not been on different sides. As the saying goes, they might have been friends if they hadn’t first been enemies. The collective trauma extends to the woman who owns the house. This is clear at the end of the story when the prisoners are shot. Their deaths bring the rebels tremendous pain. Nothing will ever be the same again, because they empathize with the men they killed.
In the poem “Babii Yar” we have a slightly different take on the collective trauma. In this case, the poet identifies with the Jews killed in the Holocaust and asks why there is no monument at Babii Yar for them. He feels their pain and suffering. The poet declares that he will fight for the Jews, even though that will make him the victim of anti-Semitic discrimination too. He fights because he empathizes.