It's the leaders who send the soldiers to war for their own reasons, regardless of what the soldier might think.
Sam was a good soldier until Atlanta, helplessly watching that pillaging thing took a lot of the fight out of him...
He fought on after Atlanta, to Franklin and beyond. So your claim that he had the fight taken out of him is a bit insulting to the man.
I guess you really didn't read the book. He said words to the effect. "After Atlanta, everything was a farce and joke."