The poem starts with a phrase we toss around without thinking: "Go and be hanged." The speaker lists the people and creatures we say it to, a clerk, a schoolmaster, two cows, God, children, and the list feels almost comic at first. It sounds like the everyday irritation of someone who snaps at whatever gets in the way.
Then the last item changes everything. The man on Shooter's Hill is not a figure of speech but an actual hanged body, with "filthy flesh shrunk to his bones." The word "hangs" is now literal, and the casual curse suddenly looks careless, even cruel.
The speaker's feeling is hard to pin down, but the poem seems uneasy about how lightly we use violent words. The mood moves from offhand and wry to bleak and sobering. The short, plain lines let the final image land without comment, so the reader is left to make the connection.