The poem is a short, sharp image of someone whose cruelty is obvious to everyone except herself. Her words don't come out cleanly. They "slip and ooze," "dribbling off her chin in thick drops," as if speech were something sticky and a little disgusting that she can't quite contain. The final comparison, "like the venom they held," tells us what those words actually are: poisonous.
The speaker seems to be watching from outside, with a mix of disgust and quiet exasperation. The key line is "Everybody but she saw," which suggests she's blind to her own effect, or maybe just doesn't care. We can't tell which, and that uncertainty adds to the unease.
The mood is cold and slightly queasy. By turning speech into something physical, like drool or slime, the poem makes unkind talk feel almost bodily, something you'd want to look away from, or wipe off.