The Civil War was absolutely about the future of slavery. Read the constitution of Mississippi, written after it succeeded from the USA. It clearly says they left the union to keep slavery. Slavery completely dominated the economies of all southern slave states.
The war was also about the southern states’ loss of controlling power over Congress. As immigration expanded the population of existing non-slave states, and led to the formation of new non-slave states, the southern slave states gradually lost their ability to control Congress. The Missouri Compromise of 1820 and the Kansas-Nebraska Act of 1854 were only temporary attempts to limit their loss of power as the USA grew.
It should be no surprise that the extreme politics over the future of slavery divided and paralyzed the nation for decades before the Civil War. Between Andrew Jackson (1829-1837) and Abraham Lincoln (1861-1865), no president was re-elected to more than one term. During those 23 years, there were 8 single-term, undistinguished, and poorly remembered presidents.
Rather than loose power, the southern states thought they could simply quit. States’ Rights was a fiction unknown to the southern states in 1860, but invented by the 1890s, when those statues were put up.
Robert E. Lee, regardless of his own beliefs about slavery, and regardless of his sense of ‘honor’, betrayed his nation by joining an armed rebellion against it. As a career Army officer, there is no doubt that he understood that. Lee was the biggest traitor to his country in our history. The same is true for all other career Army officers who joined the south in its failed rebellion.