The cause of the Civil War was slavery.

There are multiple reasons why the Civil War happened, but they can all be traced back to slavery. This older post from u/Georgy_K_Zhukov is a good starting point; it's at the top of our FAQ for a reason.

A couple notes: The Killer Angels is a novel. It's not badly written, but it's a fictional work.

Regarding the percentage of people who owned slaves, this stat is often used by Confederate apologists, but it's misleading in a number of ways: 1) most of the population of the U.S. lived in states that outlawed slavery; 2) about 40% of the population of the South were slaves themselves; 3) a better metric to understand the prevalence of slavery is to look at the percentage of households who owned slaves, since census "ownership" counted only the head of the household and not his dependents. I wrote about this here. If you look at this resource for the 1860 census you can see that the percentage of families who owned slaves ranged from a low of 3% in Delaware (which is quite an outlier) to a high of 49% in Mississippi.

(It's not surprising that the "border states" of Delaware, Maryland, Kentucky and Missouri which did not secede had slave ownership percentages of 3, 12, 23 and 13 percent, respectively -- they were not as invested in slavery as the states of the Deep South.)