To find the answer to this question, you need to refer to lines 16-19 of the poem. Basically, Kirkup is saying that we will go against nature if we take up arms against one another.
What the poet says is that we will "defile" the earth and "outrage" the air that is around us if we fight. But what does he mean by this? To me, what he is saying is that war -- the killing of other human beings -- is an act that is against nature. It is against what we as people are meant to do. So when we do it, we are contaminating the earth and the sky (metaphorically) with the evil of our deeds.
Instead of this, we are all supposed to live together in harmony because that is what nature intends for us (since we are all essentially alike).