Someone close to the speaker keeps hurting them without knowing it. The image is strange and a little funny at first: whenever they're together, this person "grow[s] another arm" and shoots. The extra arm suggests the harm isn't deliberate. It just appears, as if the other person can't help it. But the "vicious words" go "right through me like bullets," and the speaker is clearly in pain.
What stands out is how much the poem hinges on not noticing. The other person doesn't realize what they're doing, and the short final line, "Neither do I," turns the poem around. The speaker, too, seems unaware, perhaps of how deep the damage goes, or of why they keep returning to this person. It's a quiet, unsettling admission.
The mood is flat and resigned rather than angry. The plain language and short lines make the hurt feel ordinary, something that happens again and again without anyone stopping it.