What is the significance of Collin's action in turning back to give the lieutenant some water?
For some reason, the soldier Collins decides that because he is thirsty, he will cross the field that is riddled with shells where there is apparently a well near the house that is now dilapidated from the eruption of shells near it. The officers who give him permission, although nonplussed at his motivation, nevertheless allow Collins to traverse the meadow riddled with artillery if he so desires. Contemplating his venture to obtain water, Collins himself now wonders what has motivated him.
He wondered why he did not feel some keen agony of fear cutting his sense like a knife....because human expression had said loudly for centuries that men should feel afraid of certain things, and that all men who did not feel this fear were phenomena--heroes.
He was, then, a hero.
....He suffered that disappointment which we would all have if we discovered that we were ourselves capable of those deeds which we most admire in history and legend..... After all, heroes were not much.
Collins reaches the other side and hurriedly fills a wooden bucket as he now knows terror, and the canteens fill too slowly. Then, moving as rapidly as he can, Collins begins his return to the men of his company. However, as he passes the artillery lieutenant who has had his horse shot and is how trapped with his leg under the animal's body, Collins hears the officer ask him for a drink. Shouting in fear that he cannot stop, Collins, "mad [insane] from the threats of destruction," suddenly turns back to give the officer a drink amid the sudden noise of artillery, despite the look in his eyes being "all terror." Collins's act is, indeed, heroic; however, Collins is not of the "phenomena" of heroes who do not feel fear as he originally decided.
When he reaches the men, he receives a "welcoming roar." Suddenly, there is heard an oath and the bucket falls to the ground with a thud, and "a swift murmur of astonishment" because the hero has died. Despite the unselfish act of Collins, who turns back to provide the fallen officer a drink in response to his request, it is an indifferent universe that Crane portrays in his story, a universe that ignores any heroism or suffering.