Your argument is a good one...except we wouldn't have the Union atrocities documented then...they would be covered up. To some, truth is how you look at it. Some Confederate soldiers were captured and were released, after signing oaths not to take up arms against the Union again....when caught later they were executed for that violation.
Some say it was justice others say it was murder.
It is all in how one looks at it. What is their prospective.