Sassoon's "The Hero" focuses on anger, irony of situation, and hypocrisy.
Specifically, concerning anger, the "Brother Officer" is angry at Jack, a "cold-footed, useless swine." But on a larger level, the poem reveals anger at a system that perpetuates war.
A boy goes to war. He is scared to death and does not represent himself or his country well. He is blown to bits. A Colonel writes a letter to the mother of the soldier, "so nicely." A fellow soldier goes to his mother and lies about how brave he was. The mother is gullible and falls for the nicely written letter and the lies of the fellow soldier. The fellow soldier is left regretting what he's done.
The characters are all in no-win situations, ironical situations. None of the characters, with the possible exception of Jack, can be easily blamed for what they do. The Colonel and the Fellow Officer certainly can't tell the mother the truth about her cowardly son. The mother can't help but want to believe her son was brave and died nobly.
Yet, all of them are to be blamed for perpetuating the war machine. The mother, for instance, falls for the letter partly because of how nicely it is written. Those are her first words after she reads it. She bows her head because she is choking up and is embarrassed and hurt, not because she doesn't believe what the Colonel and Fellow Soldier say. She believes it and is emotionally moved.
The Colonel and the Fellow soldier perpetuate the myth of a close-knit military that cares for its own, of bravery and valor and heroism. When the truth is:
...how, at last, he died,
Blown to small bits. And no one seemed to care
Except that lonely woman with white hair.
- the Colonel and Fellow Soldier don't tell the truth to the grieving mother
- the mother believes the fabricated version of her son's military service, she believes what she wants to believe
- this is seemingly how it has to be, at least from the military standpoint
- yet this perpetuates the war machine, when telling the truth might help the public understand the truth about war and therefore make them less likely to support it
- therefore, politeness and consideration, if you give the Colonel and the Fellow Officer the benefit of the doubt, actually lead to destruction and despair.
To me, the irony used through this whole poem is that people are saying the things you expect them to say, but none of them mean it. They are all just saying things to make each other feel that the war is not really stupid and pointless. I think Sassoon is angry about the pointlessness.
You can see this in the first stanza. The mother is saying she is proud, but then she bows her head and you know she doesn't mean it.
In the second and third stanzas, we find that the officer has been telling the old lady lies about how her son died. He is telling her lies to make her feel better and he thinks it is working, but it's not.
So the officer is pretending for the old lady's sake and the old lady is pretending for the officer's sake and it is all so pointless.