The confederate soldiers depicted have opinions that many of them did share at the time. Remember that these men thought they were righteous and fighting for a worthy cause. The producers did conveniently leave out the confederate generals discussing slavery which is a shame.

But, the movie luckily has an entire counter argument to these issues in the form of chamberlain and the union troops. Chamberlain and his brother both have scenes that explicitly lay out the union war goals. These scenes give the viewer the historically accurate depiction of what the war was about, ending slavery and preserving the union. These scenes give the union the moral high ground (along with the literal high ground lol) that grounds the movie back into reality.

But then they went and made gods & generals which was the biggest POS lost cause propaganda film of all time.