The poem sets a simple scene: a field at night, empty and still. But the title, "Killing field," tells us something is going on beneath that calm. The second stanza is where the poem turns. "Who" suggests the field, or the word "good fortune," has become a person, a "she" who limps a little and carries an owl in her heart, ready for supper.
That image is strange and a bit unsettling. An owl is a night hunter, so the woman, or the field she stands for, seems to hold a predator inside her. The slight lameness hints at vulnerability, as if she might be the sort of creature an owl would catch, and yet she is the one with the owl. Hunter and prey blur together.
The speaker doesn't state a feeling outright. Judging by the imagery, the mood is quiet, watchful, and faintly eerie, with the idea that luck is scarce and nature's violence goes on unseen in the dark.