When taught about the American Civil War, we were told it was very "brother against brother" and generally clean and fair. How accurate is this viewpoint of the war? Were any massacres or inhumanities committed similar to the atrocities in other civil wars throughout the world (El Salvador, USSR, Afghanistan, Mexico)? Also, how much more complex was the civil war than simple fighting about slavery, were there further ethnic or political disputes, did these lead to any dehumanization that resulted in brutality? In short, how clean was the American civil war in comparison to those in other parts of the world?
"how much more complex was the civil war than simple fighting about slavery"
The short answer here is that it wasn't more complex than that. Slavery was absolutely the central political controversy of 19th century America. The southern states had enjoyed legislative and judicial power over the federal government for decades leading up to the election of Abraham Lincoln. With the loss of the White House to an avowed abolitionist, they realized that their stranglehold on the federal government was at an end so they revolted rather than honor the democratic process.
The single most poignant fact in the discussion of "whether or not the Civil War was fought over slavery" is that the Confederate Constitution banned the abolition of slavery. If the "State's Rights" argument is to be believed, then why would the Confederate government take away their own states' right to decide on slavery? After the War ended, southern apologists immediately began writing revisionist history that de-emphasized the role of slavery in their rebellion. Unfortunately their efforts found their way into many a history book and so today there is a sizeable portion of the American public who believe that the Confederate cause was one of self-determination and self-governance.
This is demonstrably false. The Confederacy formed because while males feared that their ability to own black people as property would be taken away from them, and they decided collectively that they would rather take up arms against their countrymen than let that happen. There are very few conflicts in human history where one side can be considered legally and morally in the wrong - the American Civil War is one of those few.