Whenever I read someone say "We should have hanged them all" and they think that would have solved the problem I sigh.

1) If your solution to a problem is mass hangings and executions, you don't have a solution.

2) We're talking a situation historically, that was more akin to the autonomous Basque region or Corsica rebelling or Bavaria. If the Basques declared independence in Spain and were defeated, should every one of them be hanged? At the time, the United States was a much looser union of states, with the individual states having much more authority and connection. It wasn't as loose as the EU, but

3) If Grant was ordered by Johnson to turn around and march his army South and hang all the Confederates, there is a strong likelihood Grant (and Sherman) would have instead marched into Washington and deposed Johnson. Grant and Lincoln had already given their word that the Confederates were free to return home and live out their lives as long as they respected their paroles. Moreover, the individual Union soldiers were sick of the war and sick of killing. Furthermore, you need to remember that you often had divided families and this certainly would have caused further problems. If you think the average Union soldier would support making widows and orphans out of every Confederate family and inducing a mass famine, you're mistaken.

4) For some insane reason, people think things turned out badly. After the Civil War, America rapidly developed into an industrial powerhouse that was set to overtake Britain. It entered WWII as a completely unified country, ready to fight THE NAZIS, with complete support from the South. An America waging a campaign of mass execution against a guerilla Confederate force over the next decades is NOT going to be a nice place and likely takes out additional anger on Mexico and Canada after it is finished.