The image you refer to as And not this man was actually published along side another image which is important in understanding its full context. They were titled Pardon and Franchise and occupied a double spread in Harpers. The first image shows southern Democrats, confederate leaders on their knees appealing to Columbia for readmission to the union. Columbia was Nast's favorite symbol to represent American values, tolerance and fairness. The was a maternal figure. When Nast includes Columbia he is making a strong point to his audience. The next image is the one you refer to, Columbia is walking alongside an African American Civil War veteran, clearly wounded and hobbling on crutches. She is standing next to him, sharing the stage with him, a marbled-columned hallway with an elaborate expensive runner/rug festooned with American imagery. She tenderly touches the veteran's soldier. She is telling her southern audience, I see you want my forgiveness, you want your rights restored, but what about him (the African American solider)? You are appealing for your rights, "and not this man?"
Nast was a radical republican, meaning in that era, that he advocated for rights for all. He was an abolitionist and pro-immigrant. He grew dejected in the progress of Reconstruction under Andrew Johnson's tenure. He felt Johnson did not live up to the vision and conceded too many compromises with southern demands. Years later, Nast had become fed up with the realities of Reconstruction and in particular was disappointed that African Americans were too conciliatory and did not fight hard enough for their hard won rights of freedom. He felt they compromised to easily with southern Democratic leaders. Johnson so infuriated Nast that the artist, who was primarily at this point simply an illustration, began to caricature the president in unflattering terms and this technique became his trademark to ridicule political enemies and players he felt were dishonest or hypocritical.
