You might like to consider the impact that the strange alter-ego that the speaker finds in the tunnel that he goes into has upon the speaker and in particular the comments that he makes about the war and the nature of being a soldier poet. According to the poem, the role of a soldier poet is to inform the public about the realities and horrors of war, which this alter-ego is unable to do thanks to his death:

I mean the truth untold,
The pity of war, the pity war distilled.

The job of the soldier poet is therefore to "distill" the pity of war and tell the truth. However, the alter-ego in the last five lines of the poem reveals his true identity to the speaker as the man he had bayoneted the day before:

I am the enemy you killed, my friend.
I knew you in this dark: for so you frowned
Yesterday through me as you jabbed and killed.
I parried; but my hands were loath and cold.

This makes the speaker realise that he himself is dead and begins his eternity at this stage. Having the speaker confronting another soldier whom he realises at the end of the poem is actually a soldier whose death he is responsible for helps to highlight the madness implicit in warfare and how men who would normally be friends and have a lot in common are left to face each other on opposing sides and kill each other.