In "Do not weep, maiden, for war is kind" by Stephen Crane, from whose point of view is the poem being told?

Crane undoubtedly drew on his experiences as a war correspondent, so we might say that the poem is written from that perspective or, more generally, from one who has witnessed war up close. 

Note that the poem is ironic and/or sarcastic. The notion that "war is kind" is clearly sarcastic because the speaker follows this phrase with descriptions of violence and death. War is not kind. The idea of the glory of fighting for one's country and/or principles is also called into question with phrases like "the unexplained glory flies above them." The poem also focuses on human loss. This includes the thousands of soldiers who have died but also the loved ones they have left behind. Mothers have lost sons, children have lost fathers, and women have lost lovers/husbands. Given these elements of the poem, one could imagine the correspondent directing his seriously sarcastic comments to them.

The point of view would come from one who has witnessed war. So, and this may be a stretch, the poem could also be interpreted as coming from the point of view of a slain soldier, offering sarcastic words of wisdom from beyond the grave.