Two short stanzas, and the whole poem hangs on the gap between what a gun is and what it was used against. The speaker looks at someone who arrived with a weapon, and the people they came for were there only for music and dancing. The title, "Disproportionate," names the mismatch plainly: a tool of violence set against something joyful and harmless.
The wording is sharp. "How little a gun" can be read two ways: the gun is a small thing, or it is a small reason, a feeble excuse for such an act. Then the second stanza turns to the person behind it. "Pettish" means sulky or peevish, and "servant" suggests someone who obeys something smaller than himself, perhaps a grievance or a bruised ego. The speaker seems to be saying that the violence answers to a childish mood.
The tone is cold, restrained, and quietly furious. There is no shouting, just a stunned, dismissive disbelief that so much harm could come from so little.