The speaker begins with something calm and almost intimate: a conversation with the night itself, an "excellent discourse" in which thoughts seem to flow freely. Then the mood breaks. A gun enters, and it has "a bullet to point with," as if the weapon were making its own argument, one that needs no words.
The title, "Monological," hints at what happens. A real dialogue, even one with the night, gets replaced by a single voice, and it's the gun's. Its "talk" is about "other things" than the quiet exchange the speaker was enjoying, and the poem leaves those things unspoken, which makes them feel more threatening.
The last couplet is the sharpest image. The gun can "undance the evening out," as though the night had been a kind of graceful movement that violence can simply reverse. The speaker's feeling is a mix of loss and unease, and the mood is hushed, tense, and quietly chilling.