Brutal and effective. In the type of verse you write, Dave -- rhyming
but without meter -- there's alway a risk of doggerel, but it works
well here. First, you don't need a lot of elaborate imagery here: as
in war poetry, simple description is the best for this type of horror.
I like that you've done that, and that such imagery as you do use
("rivers of blood, eg.) is cliched enough that it doesn't get in the
way of the narrative. Second, you've caught the voice: this sounds
like an ordinary guy (not a poet0 tossed into prison for the first
time. In that way, it reminds me of an old ballad. Who knows, 500
years from now, when prisons are just a memory, it might be studied
like one.
Your verse reminds me a bit of Robert Service, one of whose war poems
I posted for Remembrance Day tomorrow. I hope you get a chance to read
it.