Thomas Hardy's works often grapple with the indifference our society exhibits towards human suffering. In this poem about war, he shows anguish over having to kill someone that is just a regular guy, like him:
Had he and I but met
By some old ancient inn,
We should have sat us down to wet
Right many a nipperkin!
"But ranged as infantry,
And staring face to face,
I shot at him and he at me,
And killed him in his place.
"I shot him dead because –
Because he was my foe,
Just so – my foe of course he was;
That's clear enough; although
"He thought he'd 'list perhaps,
Off-hand like – just as I –
Was out of work – had sold his traps –
No other reason why.
"Yes; quaint and curious war is!
You shoot a fellow down
You'd treat if met where any bar is,
Or help to half-a-crown."
He says that if he had run into the soldier at any other time or under any other circumstances, they probably could have had a normal conversation, they probably could have shared a beer. And yet, because of the absurdity of war, two regular guys find themselves face to face, enemies, one having to kill the other. He regrets having to shoot the other man and if it were not for war, he would not have been forced into having to take another life.
Read about Thomas Hardy here on enotes.
The speaker's attitude or tone in Hardy's "The Man He Killed" is one of irony.
The speaker reveals the irony involved in war. A man one might otherwise lend a few bucks to or buy a beer for, becomes your enemy when on an opposing side during a battle.
The irony is added to by the speaker when he points out that the opposition probably isn't fighting for anything in particular, either. According to the speaker, no lofty ideals are involved. The opposition probably just enlisted because of some mundane reason, like not having a job.
Yet, when facing a man who is similar to yourself during a battle, your job is to shoot this man who is just like you.
That's the irony of war in "The Man He Killed."