we can all agree that slavery was terrible

Unfortunately, not everyone does agree. There are a fair number of Americans who consider the Confederacy to be "their team" and defend every aspect of it. Some push their line via reddit. So redditors have become a bit cautious in their replies about slavery.

As for your questions, the comparison between a slave and a car is only apt in its narrow scope: cost. Comparing the destruction of a slave and a car doesn't work unless the car is Christine. Ultimately, slave owners killed slaves because they were afraid. I think you know of the Haitian Slave Revolt of 1791, and the many smaller revolts in the Caribbean and the US? How much profit would you forgo to prevent that?

Banks still take human lives as security today. What do you think happens to a college loan if the borrower dies? Credit card debt? The higher death rate among slaves than modern citizens was factored into interest rates. Even the most murderous slave owner wouldn't kill enough of his slaves on average to make it a bad investment.

Yes, slaves could be repossessed, and yes, emancipation did destroy many fortunes in the South.

Were the slave owners really so wealthy that they could kill something worth the cost of a modern car for no good reason? Even most modern wealthy people don't go around destroying $30,000 cars for fun.

Now, this analogy is going to offend some, but bear with me. Imagine you are a farmer who suddenly discovered that one of your livestock has a deadly and contagious disease. Are you going to try to nurse that animal back to health? Not likely. The not insignificant loss of a single animal is still better than losing a whole herd. Now, if you're a slave owner in 1850 Georgia, sure, a slave is very expensive to replace. But if killing that slave is going to serve a purpose, such as cowing the other slaves into obedience, you do it. This also assumes we are talking about rational actors here. You also have to consider that people are not always perfectly rational. How often do we see even the very wealthy make poor decisions with their property? Here's a simple but effective example: https://www.google.ca/webhp?sourceid=chrome-instant&rlz=1C1ASUC_enCA586CA586&ion=1&espv=2&ie=UTF-8#q=wrecked+lamborghini&tbm=vid

Using slaves as security to back loans seems like very unwise collateral. A simple farming accident could render the security completely worthless. Present banks loan money on real security such as land and buildings which are obviously far more secure than a human life. Why would a bank in the 1800s even consider such a risky form of security?

This may surprise you, but even today you can get loans on the backing of basically nothing. Have you ever been to a payday loan store before? They are giving you a loan whose only backing is a paystub. Think about that for a moment. Now, go back to 1800s America where the banking sector was much less refined and much riskier. Why would a bank loan that money? Because they thought they could make money on it.

After the abolition of slavery, I assume that the value of the security dropped to zero. Were there massive defaults? Did it bankrupt many once rich and powerful families and companies? Or, did the banks and financial institution simply write off the losses now that their collateral was worthless?

Most of the banks servicing these loans would be local, and suffice to say, the entire economy of the South was in severe trouble after the war. The defaults on the slaves pale in comparison to Sherman's despoiling of Georgia, let alone the currency crisis in the South, the destruction of infrastructure, etc. Basically, what I'm saying is that it was a big problem, but in general much of the South saw its economy essentially destroyed. Banking failures from slave debts were only one cause.

Took a history of slavery course in university.... I remember seeing that almost half of all slave owners in the American South actually were poor farmers, and owned only a few slaves (less than 5). The great plantation style farms with dozens of slaves were NOT the norm, and were a minority. However, most of this wealth accumulated at the top of the social strata. These large plantation owners carried a lot of political weight and influence, which in turn led to the banking practices you speak of. I remember reading that the economics and the social/cultural realities of the South could not be divorced. Although the economics did not always make rational sense, it was very much a part of Southern society.

Frederick Law Olmstead travelled the South and wrote extensively on his experiences. One slave owner he met actually despised slavery and was morally repulsed by it, but he saw no other way to earn a profit on his plantation. In the end this owner said he wanted to free his slaves and return them to Africa, where he felt they belonged. This is why some historians called slavery the "Peculiar Institution".