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.

How does the quote illustrate the collective trauma of war?

"The veterinarian watched in numbed silence, overtaken by the sense that he was beginning to split in two. He became simultaneously the stabber and the stabbed. He could feel both the impact of the bayonet as it enters his victim's body and the pain of having his internal organs slashed to bits."

The key to understanding how this quotation reflects the collective trauma of war lies in analyzing the thoughts of the veterinarian. As a vet, his peace-time occupation is healing, helping animals and their owners by repairing injuries. In this passage, however, he is not acting as a vet but as a soldier, and not just a passive soldier but an actively attacking fighter in battle.

The soldier is using his bayonet to stab his victim and is feeling the sensation of the knife entering the victim's body. He has probably used a knife to cut into a body before, as a veterinarian operating on an animal he is treating. He understands what it means to cut into a body - internal organs are "split in two" as the knife cuts into the flesh.

But never before has the veterinarian violently cut into human flesh. The experience is traumatic because he fully understands and identifies with what he is doing to the enemy's body. As a soldier, he is following orders and doing what is required. As a veterinarian, he is violating his training and instincts, and his mental reflections indicate the inner conflict that is resulting.