So what is the theme of the poem? What is the main message that the poem is trying to convey to the reader?

One of the messages the poet is trying to convey is the omnipresence of violence. Each stanza contains a simple image that becomes the springboard to the narrator's development of that image into something metaphorical. For example, when the narrator walks by a butcher's shop and sees a single light, the narrator thinks it's like the light with which a convict digs a tunnel to escape. In the second stanza, the narrator sees an apron smeared with blood, which is compared to "great continents of blood." In this image, blood is smeared across the world, implying that the entire world is stained with blood and marred by violence. Other images include a wooden block on which bones are smashed. These images of the butcher shop support the theme of the poem--that violence lurks everywhere, even beneath everyday objects.