I was a little annoyed by Capitalism Destroys Slavery because it seems like intentionally losing an argument. The correct answer, in my mind, is that capitalism explains slavery, as well as it's subsequent eradication.
Slaves were capital goods. They required a lot of input to keep them alive, so they were costly capital goods. The innovation that occurred during the industrial revolution led to it being economically unsound to use slave labor. Now, the technological innovations of the industrial revolution did not reach everywhere at once, and in the particular case of the U.S., the North used it's advance technological capacity to make war on the South, and used the evils of slavery to excuse themselves.
How do we know slavery is evil? Well, basically people reflected on capitalism and derived a moral argument from an economic one. The economics are value free in the sense that it is an endeavor to explain how humans allocate resources in a world with finite resources in it. The idea of self-ownership is a relatively new one, and is still actively fought often by precisely the sort of idiots who would think slavery is a capitalist institution.
What happens is that people who are successful, often through capitalist means then find themselves in a position to secure unfair advantage via government. Thus slavery, or at least the body of law that perpetuated it past it's prime, was a collectivist institution. When slavery was got rid of, industrialists happily used the same influence to treat their workers worse than they should. The workers, the poor, and the downtrodden, don't have good leadership skills, so whenever there's some sort of revolt in their name, there are usually a bunch of bloodthirsty academics actually running things- except maybe Venezuela. Chavez seemed like a working class guy, and if the U.S. weren't so wedded to big oil interests, we may have been able to work with him. I mean, the guy did't understand reality, but he wasn't like Che.
The distinction isn't made though, so capitalism is plainly thought of by most, even those who agree with Don Boudreax, as part of a subset of 'isms' which can have institutions, rather than an explanation. Additionally, the desire to paint capitalism as modern is dangerous and reinforces the idea that it is an 'ism' that just sort of showed up around the same time that socialism did. The idea certainly did show up rather recently, but it explains the past too. People had fewer forms of capital goods back then, but when they were allowed to they would go through the same process of saving to buy capital goods and then put those capital goods into production. Indeed, people had rather more good sense back then and tended to give capital goods to young couples so that they could provide for themselves rather than run them into debt for a worthless college degree.