Always a pleasure reading your work.
Alliteration, from my experience, is not common in poetry. Again, I am speaking from experiece, alliterations usually become a little silly. This avoids that trap altogether, thankfully.
You're also right. Stylistically, is is a little rough around the edges, but in this case, "rough" may not be such a bad thing. It doesn't draw attention to itself, which is good, and it's near-stifling nature reflects well with the subject matter.
And before I move on to the actual advice, you may already know this, but bear in mind that not being able to catagorize a poem can be a good thing. Poetry should thrive on breaking boundries, and what is a catagory other than a big, ugly boundary?
NOW, let's talk about the actual poem. The first stanza talks about a force of good, something that drowns out evil. The "drain" comes into play in the next stanza, where combined with "depths" and "rain," we can infer that the setting is a sewer. (I'm only trying to go over everything, sorry if I give some "duh" details). The flood mentioned sounds like another cleansing force, and it takes out all the rats living there. For the final stanza, morning has become mourning, suggesting that maybe this "cleansing force" was a little short sighted.
I love the idea that wiping something out completely, regardless of how it's viewed may be a shortsighted move. What seems to be missing is the actual repercussions. This is where style clashes with subject. While the poem is a good length, I kind of wish there was a mention of WHY this move turned out to be bad. With the addition of that "why," this poem becomes something of an instructional fable. Add the "why" and you risk sacrificing the "length." I know it seems nitpicky, but it's difficult to find anything wrong otherwise. I hope this was helpful.