By May 1806, Charles Dickinson had published an attack on Jackson in the local newspaper, and it resulted in a written challenge from Jackson to a duel. Since Dickinson was considered an expert shot, Jackson determined it would be best to let Dickinson turn and fire first, hoping that his aim might be spoiled in his quickness; Jackson would wait and take careful aim at Dickinson. Dickinson did fire first, hitting Jackson in the chest. Under the rules of dueling, Dickinson had to remain still as Jackson took aim and shot and killed him. However, the bullet that struck Jackson was so close to his heart that it could never be safely removed. Jackson's behavior in the in the dual outraged men of honor in Tennessee, who called it a brutal, cold-blooded killing and saddled Jackson with a reputation as a fearful, violent, vengeful man. He became a social outcast.More here:
Though acceptable by the code of the times, many people considered it a cold-blooded killing. I presume the rules of engagement were for each man to draw and fire at the same time, upon hearing the signal, but if one fired, there was no "second round" until the other man fired. The implication is that magnanimity would have required Jackson to fire into the air rather than taking a slow deliberate aim at 24 feet.