"If matters were reversed, he would do the same for me." He died about ten days later".
There were many friendships among the officer corps of both side from West Point and the Mexican War days. Respect for a person as a person can sometimes be detached from the actions of that person.
For example a man commits murder. But his mother still loves him and defends him. That doesn't mean that the mother approves of the fact that her son has murdered someone.
War is War. The purpose of an army in war is to break things and kill your enemy.
Now granted the "Breaking Things" part involves anything that can be used by your enemy to keep his army in the field. Which pretty well means everything that your enemy has.
But in civilized warfare for the last 300 years among civilized european nation, the unwritten code (until it was specifically layed out in the Geneva Convention) was that you did not wantonly destroy or wage war against the civilian population unless it was absolutely necessary.
A lot of destruction caused by Sherman's army in Georgia had no military purpose as the basis. It was simply wantonly destruction, murder and theft for the sake of despotism and retribution alone.