'Oh! Jesus Christ! I'm hit,' he said; and died.
Whether he vainly cursed or prayed indeed,
The Bullets chirped-In vain, vain, vain!
Machine-guns chuckled,-Tut-tut! Tut-tut!
And the Big Gun guffawed.
Another sighed,-'O Mother, -Mother, - Dad!'
Then smiled at nothing, childlike, being dead.
The machine guns are mockingly disapproving of the human’s activities. ‘Tut-tut!’ Moreover they ‘chuckled’ —  another mocking understatement; killing is no laughing matter.