"Helen" is incredibly sound-y. And by "sound-y," we mean awesome to listen to. It's got tons of repetitions, a big bunch of rhymes ("smiles" and "reviles"), a big pile of assonance ("hates" and "face"), and a whole heap of alliteration ("wan" and "white"). There are all of these repeated sounds bouncing around the poem. And they're bouncing around a bit haphazardly because there's no regular rhyme scheme in "Helen."
See, H.D.'s not after some fancy structure or sing-songy feel. Instead, when we read "Helen" aloud, we feel a bit like we're in an echo chamber. A really small one, that makes us a bit claustrophobic. The same sounds keep repeating and bearing down on us and we can't escape it all until we're dead.
Oh wait, that's Helen. Well, us and Helen. The poem's repetitions suffocate us as the constant hatred effectively suffocates Helen. Once again, form and content go hand in hand. Ain't that nifty?