I'm not a historian, but this is a pretty easy one to answer.

most of the habitants (including people like Jefferson Davis) actually treated their slaves as equal human beings - they weren't exploited, but rather employees bound for life.

This is certainly not true. Blacks were not citizens, even when free, so they certainly could not be "equal human beings." As slaves, they were not entitled to pay, and subject to incredible punishment for nearly any purpose. Their families could be separated by sale, and their children were slaves as well, merely by virtue of birth. Slaves, both male and female, were subject to rape and sexual abuse without any real recourse or protection. An escaped slave could be recaptured anywhere in the US and returned to their master by force of law. During the Civil War, Southern armies entering Northern territories captured and enslaved free blacks as a military tactic, shipping them south to be sold for work or to fund the war by their sale.

Slavery was a brutal, shameful institution. Anyone who tries to sell the story of the noble, kind slave-owner is cherry-picking their examples, lying, or has been lied to and is too lazy to do the debunking research.

The southern United States, along with the West Indies and parts of South America, practiced what is known as chattel slavery. This means that slaves have no more rights than a cow or a pig, and that their treatment is entirely at the whim of whoever owns them. Thus we see a laundry list of abuses: beatings and floggings, which most of us are familiar with, but also systematic rape and impregnation of black women, the sale of children, very poor living conditions, etc.

To support this concept, very complicated theories of racial inferiority were developed, so that free blacks, who were much rarer than some revisionists would have you believe, were scarcely treated any better. Furthermore, Southerners, who, in certain regions such as the Mississippi Delta, Tidewater Virginia and North Carolina, and Lowcountry South Carolina, were heavily outnumbered by black slaves, were quite terrified of the idea of slave revolts. They passed repressive measures to discourage them, such as outlawing the teaching of literacy to slaves, making it more difficult for masters to manumit (free) their half-black children, et al. When the slave revolts occurred, retribution out of scale was the rule. After the Stono Rebellion of 1739, 80 or so slaves were killed in battle or executed, and their heads were mounted atop mile markers on the road to Charleston. After Nat Turner's revolt of 1831, 100-200 blacks were executed.

Furthermore, in Southern society, the possession of slaves was as much a mark of social distinction as the owning of a factory in the North. Most slaves were not kept on vast plantations, but were owned by middling farmers, or urban professionals. Having slaves meant that you were not a mean laborer or a subsistence farmer, which no white man in the South wanted to be. Children starting out were frequently gifted a few slaves by their fathers.

McPherson, James. Battle Cry of Freedom. William C. Davis. Three Roads to the Alamo.

There any any number of qualifications we can use to judge "how bad" the Confederacy was, but you are asking a subjective question. I mean, if we accept that slavery is evil, and further accept that the South was fighting the Civil War due to their fears that the institution was being threatened - See this post I did earlier today - well, that speaks poorly for them, doesn't it? (Yes, we can look at them relative to the time period, but even then, slavery was starting to become the red-headed stepchild of America as far as many in the North were concerned).

As for the treatment of slaves, well, /u/anewmachine615 already touched on some of the nature of abuse. One specific nature of abuse I'm going to address though is sexual abuse. Now, to preface, I don't want to get into semantics of what rape is here. Even if a slave was going along with her master in giving into his sexual demands, the nature of their relationship is fraught with issues, so lets just agree that whatever the nature of consent was, there is something unseemly going on here to say the least.

Anyways, the rise of cheap, easy DNA testing has been a real boon for people looking into their roots, and also has given us some very interesting peeks into history. A little while back, TheRoot.com published a piece summarizing the findings of mass-market DNA testing companies from when they analyzed African-American DNA.

What did they find? Depending on the company, the average percentage of DNA in African-Americans that was of European ancestry was between 19 percent and 29 percent. For the patrilineal line specifically (father's father's father's etc), 35 percent of African-Americans would eventually hit a white ancestor. And of course that doesn't account for maternal great-x-grandfathers. If we assume that the white DNA is almost exclusively coming from a male ancestor (not to say it couldn't happen where a white woman slept with a black man, but I think it reasonable to assume it happened less frequently), than we can double those numbers, and say that the average African-American's forefathers are very white. Between 38 and 58 percent in fact. I would think the implications of that are clear, but just to drive home the point, a LOT of that comes from masters or foremen having sex with African-American slaves, possibly in violent circumstances, but almost certainly as part of a coercive relationship where they couldn't exactly say "sorry, I have a headache tonight".

While there is certainly some degree of pluralism to this- multiple things going on at the same time- it was often the case that slaves were treated....poorly. I mean, it wasn't as bad as, say, Christopher Columbus forcing local populations to work to the bone so that they'd get a voucher which made sure their hands weren't cut off, but it was still pretty bad.

These people had little to nothing in the way of rights, they were often outright banned from getting an education, and typically treated to a morbidly precise science of how much could be done to keep them alive while also beaten and suppressed.

Were there exceptions? Sure. Only around 1% of the US population even engaged in the act, and every subsequent decade that brought the nation closer to the civil war saw fewer and fewer slaves brought into the country. Most evidence points toward Robert E. Lee being a respectable man who, while technically a slave owner, disliked the concept, and treated his fairly to the extent of ignoring some laws.

Furthermore, while the issue of slavery may have been a front and center issue for the Civil War, its not hard to argue that the slaves themselves were not. But history won't furnish you with an example where a large sector of the populous- around 1/5 of the south was slave- is suddenly turned loose with no education, widely illiterate, and only having experience working as slaves. At least they didn't commit the blunder that the Russian Empire did when it freed the serfs, but then turned around and told them they had to effectively buy themselves from their masters.

Long story short? The south was no Gone with the Wind or Song of the South. Slavery was repugnant, and the wider majority of it was conducted in a way that was inhumane, and stressed the fact that slaves were property. At the same time, anyone trying to throw around terms like "evil" you should be suspicious of. History is more complicated than that. Then again, I'd be wary of people who try to portray people from the south as the Southern Gentleman. While it isn't like such a thing never happened, you'd be prudent to do your own research and reach your own conclusions. The South, Slavery, and the Civil War were all complicated things.

You really cannot explain away the Civil War as something people actually fought over what an astonishing 1% of the population was engaging in, many of whom didn't even live in the south necessarily. President Lincoln writing letters assuring organizations that the war was not about slavery, in fact, is a matter of public record. If you want a good, short, and dirty overview of just how badly skewed our view of Antebellum South and the civil war is- coming from both the angle of historic revisionists, and those who really want to drive home that the South was, collectively, the children of Satan- read "Lies My Teacher Told Me." It's actually a good critique of everything we get wrong with history classes in the US.