What is the message of the poem?

"Suicide in the Trenches" is one of Sassoon's most famous war poems. It confronts the grim reality of war with characteristically blunt and unflinching language: we are left in no doubt as to the manner of the titular suicide when Sassoon tells us "he put a bullet through his brain."
The abruptness of this line echoes, in itself, the abruptness of the shift from life to death after a gunshot. The juxtaposition between the tone of the second stanza and the first is stark: the "simple soldier boy" of the poem was once cheerful enough to have "whistled early with the lark" and "slept soundly" through the night, his life filled with "empty joy." The words "empty" and "simple" suggest that the boy, an unnamed everyman, previously had very little to worry about. The shift from this "empty joy" to suicidal ideation has been occasioned by one thing, and one thing only: war.
The "moral," or message, of this poem is presented to us straightforwardly in the third stanza, which offers a sort of summary, or conclusion, to what has come before. The soldiers in the "winter trenches" have no recourse to joy, ridden as they are with "lice and lack of rum." The speaker does not feel the need to emphasize why, exactly, these "winter trenches" would drive a man to suicide. The speaker knows the reason, but the implied listener, the "smug-faced crowds with kindling eye," will never know it. Indeed, they do not even attempt to ascertain it: having committed suicide, "no one spoke of" the soldier again. Suicide was considered the coward's way out, something taboo which should not be discussed.
Meanwhile, the "smug-faced crowds" continue to blindly applaud the exploits of soldiers and "cheer when soldier lads march by." The poet suggests that these people should "sneak home"—the word "sneak" implying that they should be ashamed of themselves—and "pray" they will never understand "the hell where youth and laughter go." That hell, which has turned a "simple soldier boy" into a suicidal waste of young life, is the hell of the winter trenches, an unending horror. Instead of confronting this grim reality, the general public would rather sweep the suicides of young men under the rug and pretend that war is something to be cheered.