The poem is short and fragmented, and its mood is unsettling. The speaker seems to be asking for instructions on how to behave, and perhaps how to feel, in a terrible situation. The odd line "which neither denied nor affirmed hit / not to see" is hard to pin down. The word "hit" looks like a typo or a deliberate slip, and the line suggests something that neither says yes nor no, a vague answer that leaves the speaker on their own with the choice of not looking.
The second stanza makes the situation concrete and disturbing: the speaker wonders which "good times" they could call to mind at a public hanging. The idea is to distract oneself with pleasant memories while watching something brutal.
The title, "Model citizen," seems ironic. A good citizen goes along, attends, and perhaps looks away. The speaker's feeling seems to be uneasy complicity, a mix of discomfort and numbness. Judging by the text, the poem asks how easily we retreat into private comfort when we ought to face something cruel.