Three short lines carry a lot here. The poem opens with "unseasonable light," a phrase that sounds almost pleasant, like a warm glow arriving at the wrong time of year. Then the next line undercuts that calm: "war, arson, a city." The light, it seems, isn't a gift of nature at all. Judging by the title, "Against nature," it is something unnatural, perhaps the glare of fires or explosions lighting up the sky where no sunlight should be.
The speaker never says "I feel," and that restraint is part of the effect. The list of nouns is flat and clipped, as if the speaker is too stunned to explain, or is simply reporting what the eye sees. "A city" ends the poem without a verb, leaving it hanging as the thing being destroyed, or perhaps just the place where all this happens. The mood is cold, uneasy, and quietly horrified. The poem suggests that human violence can distort the natural world itself, even bending light into something wrong.