Is "The Hollow Men" written by T.S. Elliot a warning about coming events or a reflection of a situation that has already developed? Does "The Hollow Men" seem to be more didactic or reflective?

In my opinion, "The Hollow Men" is both. All of Eliot's poetry is a commentary on things already happening in early twentieth-century social and intellectual life. Alienation and despair are the frequent themes of literature in the period of World War I and its aftermath. The ghostly images of Eliot's poem are a description of a culture that has essentially lost its way and believes in nothing.
In any artistic movement, it's difficult to know for certain whether the artists have created the Zeitgeist, the spirit of the time, or are merely reporting on it, reflecting it. The answer is that both are true. Artists influence the outside world and are influenced by it. It is a circular, self-perpetuating process. Eliot's "The Love Song of J. Alfred Prufrock" was written in 1910–1911, before World War I had even started, though people usually believe that the War was responsible for the negativism and disillusionment permeating European (and ultimately American) thought which "Prufrock" expresses. "The Hollow Men" is a kind of extension of the horror of the Great War. It is even reminiscent of Wilfred Owen's "Strange Meeting," in which a dead soldier who says he is in hell speaks to the man who shot him dead. "The Hollow Men" takes place in "death's dream kingdom." At the same time, it's a commentary on the peacetime world in which people no longer believe in the old values.
All art is both a commentary on the real world and a prediction of the future in the sense that art, by definition, creates its own world which is an alternative to photographic reality. The end of "The Hollow Men" clinches this vision of the future in which Eliot predicts "the end" as a resigned—just like his era of the 1920s—dying out, "not with a bang but a whimper."