The speaker's tone is one of judgment, disapproval, and disdain for any individual or society that would tell its young people that "it is sweet and becoming to die for one's country," as the ancient Roman poet Horace wrote. The line that best conveys this tone is the penultimate line of the poem, in which the speaker describes this statement as "the old Lie." In this line, if we didn't already figure it out, we learn that he believes war is horrible, grotesque, and certainly not a "sweet" way to die at all.
Throughout the poem, the speaker seems to address an individual who would echo Horace's sentiments, calling that audience—perhaps with a little verbal irony—"my friend." The speaker describes the horrors of war: how it turns young men into "old beggars," seeing a fellow soldier fail to get his gas mask on before inhaling the poison, watching as the man slowly and painfully dies from his internal injuries, and the nightmares that follow the fighting. He insists that if his audience could see these horrible sights for themselves, hear "the blood / Come gargling" from the gas victim's burned lungs, and taste the "bitter[ness]" of the "vile, incurable sores" that gas has made on the young man's tongue, that audience would never tell young men, "Dulce et decorum est pro patria mori."