Answer :
kissing a black man.
guilt at doing an "unspeakable" thing, "kiss[ing] a black man" (20.45).
"She did something every child has done—she tried to put the evidence of her offense away from her. But in this case she was no child hiding stolen contraband: she struck out at her victim—of necessity she must put him away from her—he must be removed from her presence, from this world. She must destroy the evidence of her offense. What was the evidence of her offense? Tom Robinson, a human being." (20-43-44)