In this poem, the speaker seems to suggest that the world, after the first coming of Jesus Christ, has gone terribly wrong. "Things [have] fall[en] apart" and "anarchy" has been "loosed upon the world." There have been horrible wars and destruction, and the world seems to have lost its innocence, especially after the Great War, what we now refer to as World War I. The best, most moral people "lack all conviction, while the worst / Are full of passionate intensity." The good do not feel strongly enough to act on their morals and effect change, while the bad work passionately. What should be good is bad and what is bad is good; nothing is what it should be considering the peace and love advocated by God.
In the second group of lines, the speaker considers the idea that "the Second Coming is at hand," and it will not be Jesus but, rather, some monstrous animal with "A gaze blank and pitiless as the sun" who comes because this is what we deserve. We have made a hell of this paradise, and those "twenty centuries of stony sleep" will now result in a terrifying and dreadful punishment. This monster, this "beast" that "Slouches towards Bethlehem" is unknowable and unpredictable, especially because we so deserve the consequences it comes to deliver. The poem ends with a question because we cannot know this monster or the punishments it will inflict upon us. It's like a cliffhanger: it will be bad, but how bad?