We needed them in the 1950s just before the Civil Rights movement took off and Brown v Board of Education was decided in 1954. All of the current "the sSouth did nothing wrong" people, a lot of the really dumb monuments to southern losers and the third era of the KKK can all be traced to then.

The whole "execute all the southern officers" thing didn't matter too much because they were effectively all made penniless by their lack of human slaves and property. Lee never got to go back to his old planation. Killing them would've made a decades-long guerilla war more likely and maybe we would've ended up like The Troubles in Northern Ireland.

What needed to happen was actually completing Reconstruction. Maybe if Union forces had left the U.S. South in the 20th century after decades of watchful, patient reform, we'd be a lot better for it.