Owen is writing about the young but exhausted, battle-worn soldiers on the front in World War I when he begins his double sonnet with the description that they were "Bent double,like old beggars under sacks." He likens them to old men--beggars--filthy, without much hope or ambition, trudging along mechanically under the weight of their packs while the battle rages behind them.
When he describes the mustard gas attack, he describes the man who doesn't get his mask on in time as one "flound'ring like a man in fire or lime." Lime is used to decompose bodies faster, so if put on skin, it burns like fire. A man in fire flounders, swinging wildly and crazily to get away from it.
After the man is too far gone to help, they throw him into a wagon. The narrator speaks of walking behind him and seeing "His hanging face, like a devil's sick of sin." The man has died horribly and Owen describes something we cannot connect with on a concrete level since we don't know what a devil looks like, really. We can only imagine it's horrible. He uses this simile, though, to connect the man's meaningless death with his condemnation of the war. He himself is sick of sin (the sin of the war), but this image is of a devil--one who thrives on sin--but who, Owen suggests, has glutted on it, as well.
The last two lines translate roughly to "It is sweet and glorious to die for one's country." He calls it a lie because it is the line used since antiquity to convince young men to enlist and go to war, but from what he has seen on the front lines, there is nothing sweet and fitting about it. Quite the opposite: death in war is horrible, pointless, and completely random. It has nothing to do with heroism.