Stephen Crane calls war "kind" because he mocks with bitter irony the chauvinism that would have young men believe that going to war is noble and dying for one's country is heroic.
For those in the military, Crane ironically declares that they are meant for death:
These men were born to drill and die.
The unexplained glory flies above them,
Great is the Battle-God, great, and his Kingdom--
A field where a thousand corpses lie.
As a writer of the Naturalist school of thought, Crane takes a rather deterministic view of the dying of soldiers in stanza 4 as he points to the "virtue of slaughter" and "the excellence of killing," satirically implying that since the soldiers are destined to die, war accomplishes mass deaths and is an efficient way to end many lives.
Thus, the reader perceives that Crane really feels that war is anything but "kind"; it is an outrage against the youth of soldiers, stealing them from their mothers and lovers, only to be buried in "A field where a thousand corpses lie."