I do not think there is any way to doubt that Millay is opposed to World War II in this poem. She seems to be opposed to all war, given her words here.
I suppose you could argue that she is expressing support for the war because the words of the poem are literally cheering war on. But the words are not, I think, meant to be taken literally.
In pretending to be cheering war on, she is really just listing all the terrible things that war does. I do not think she is seriously cheering for Homo sapiens to be wiped out as a species.