The speaker begins with a confession that sounds almost like a defense: they have never killed anyone directly. But the next line takes that comfort away. Harm can also come from neglect and cowardice, from what we fail to do. The speaker feels guilty, and wants to be punished for it, yet finds that such failures are rewarded, or at least go unpunished.
From that observation the poem draws a bleak conclusion: the world is "a dreadful place." The speaker then offers a sharp idea about religion and morality. Gods, heaven, and even the word "justice" may exist because reality doesn't deliver justice on its own. We invent them to fill the gap.
The mood is quiet, tired, and sober. The short lines and the break before "this" make the speaker seem to stop and face an uncomfortable truth. The final image of people "snatching" at words suggests desperation, as if justice were something we grab for because we need it, not because we can count on it.