I do agree - things shouldn’t turn out that way. I entered some discussions here to find out how much of (unnecessary) destruction happened but could never get a clear picture.
I am pretty much convinced that the US government in reality acted quite lenient during reconstruction - but it might be possible that much unnecessary damage was done during the war.
You are not alone in your views. In the October 2012 issue of Civil War Times, scholar Stephanie McCurry, wrote:
The level of barbarity and violence reached in the United States pales even in comparison to the other major example of a civil conflict fought conventionally. In the Spanish Civil War (1936-9), there were, in addition to 300,00 battlefield deaths, at least 200,000 extrajudicial killings of civilians-including the purposeful killing of many women and children behind the lines. Three-quarters of them were killed by Franco's forces in mass executions... More than half a million refugees were forced into exile, and many died in French concentration camps... There is little in the American record to compare to this systematic targeting, terrorizing and exterminating of civilians for purposes of political repression...
It is a sad truth that the civil wars of our own time constantly force the Civil War into new perspective. Observers of recent genocidal wars... are unimpressed with the violence of the American war. What strikes them most is the level of restraint observed by Union troops in their treatment of enemy soldiers and civilians. What other country, they ask, adopted rules of war in the midst of the fighting? Indeed. It is one of the most impressive and - yes - unique features of our war; that the Lincoln administration was willing to bind itself to a set of regulations limiting the latitude of the Union army in its operations, including in occupied territory and guerrilla warfare. It says something profound.
The post war era was also marked by this relative restraint. In his book Reconstruction: A Concise History, Allen C. Guelzo stated
Reconstruction followed the route of generosity—it created no conquered provinces, no mass executions for treason. As Walt Whitman wrote, almost in self congratulation, Reconstruction “has been paralleled nowhere in the world—in any other country on the globe the whole batch of the Confederate leaders would have had their heads cut off.” Ironically, most of the violence that pockmarked reconstruction was inflicted on the victors, not the vanquished.
None of this will mollify many viewers of this period, they will not buy it that the US was "relatively" restrained. But comparative analysis of world history can lead us to this conclusion.