how does wilfred owen create futility in war in dulce et decorum est

In all my dreams, before my helpless sight,
He plunges at me, guttering, choking, drowning.

these two lines form their own stanza. They Should be part of the second stanza.
We don't have a solid lock on Owen's intentions here, of course, but here's what the poem itself tells us: this stanza fits into the rhyme scheme of stanza two.
In fact, it's almost like Owen snapped it off of the second stanza and pushed it down the page a little ways. these two lines bring us out of a past experience (the experience of the gas attack) and into a horrific present.In some ways, the present is a lot like the past – after all, all our speaker can think about is the gas attack.
In others, however, it's a marked shift in the momentum of the poem.We can't think of the dying soldier as part of the past, if only because he plays such a huge role in our speaker's present.
"All" his dreams have been taken over by a nightmarish memory of the gas attack.Notice now how the speaker seems to be directly involved in the man's suffering: in lines 14-15, watching through "dim" light as his comrade goes down.By the time we get to line 16, however, the other soldier "plunges" directly at our speaker. Moreover, the helplessness of our speaker takes center stage.
He can't do anything. He can only replay the horrors of the scene, turning them over and over in his mind.It's almost as if using the word "drowning" at the end of line 15 triggered our speaker's memory, making him re-hash the horrors that he's witnessed.
"Drowning" occurs again in line 17. In fact, it actually rhymes with itself.