Death is all over "London"—literally. The title of the poem may well have been "London: the Dead City." Hearses, bloody palace walls, blights, and plagues—death is everywhere. In this poem's universe, all this death is the result of war-mongering governments ("palace") and corrupt institutions like the church ("blackning church"), which allow child labor, prostitution, and war. Things that formerly promoted life and unity, like marriage, now only create more death ("hearse"). Sadly, the picture is bleak—there seems to be no end in sight for all this death, a fact evident in the poem's extremely repetitive structure, word choice, and tone.
Youth is associated with death, rather than life, in this poem. This is totally backwards, which is the speaker's way of saying London is totally out of whack.
"London" is full of vampires. No, not the Dracula kind. Everybody is literally alive, but they're also really, really dead—inside.