Embers

[brynmawr1]

A slash like smile,
cleaves faces in two;   I agree that it reads better with different punctuation.  I like the imagery of the first two lines.
Predators
bait the killing room.

Like freaks at a carnival,     Also agree that this can be trimmed for better effect.
with words that disembowel.
Feigning to be civilized,
"O how politely they growl.     This is good in to context of the stanza and the juxtaposition of "politely" and "growl".

A nightmare breathed life,    try to shorten here to one word.  Maybe "incarnated"?
Like true monsters Grimm.
Waiting in the shadows;
torturous, is their offering.  i don't like "torturous" here.  it's an adjective maybe modifying offering but too awkward, discordant.  But I like the phrasing.

The darkness is unbending   I think you are going for something like "relentless".  Need new term and shorten. "Darkness unbending"
As like blackest obsidian.     two simile terms here, pick one or go another way.  may be just "the"
Fires that once burned;
now embers,
flicker and dimm.  I like the double "m" to connect with Grimm.  Didn't notice is consciously at first.  Probably unnecessary, but I like it.

Overall I like this poem.  I think the last two stanzas are the strongest and I think it ends strong.  I am not a fan of gore type poems, but that's me.  One other mention is that I am not sure exactly what your message is, but again, this isn't my thing.