If you're against slavery and feudalism, you should be against pure unfettered capitalism (though possibly in favor of almost-pure capitalism, I guess).
tl;dr: "Pure capitalism is good", as a problem-solving strategy, is not entirely trustworthy; sometimes it leads to horrible horrible things.
The fundamental doctrine of capitalism is that it is just (not merely pragmatic) to let the people with the gold make the rules. This, of course, is the single biggest point of The Wealth Of Nations, and it's an interesting one, and seems to be largely correct. But if you take this to an illogical extreme, you are back in might-makes-right.
I think it's pretty obvious that slavery wasn't defeated by capitalism, but rather by a leftist people-other-than-the-rich-have-rights kind of movement. Master Smith had the capital, he bought the slaves, and now we're going to deny him his property rights, because someone else's rights (to freedom and personal security) are more important.
Feudalism is also basically free-capitalism-plus-ethics. If you conquer some land by the sword, you get to keep it. There are some people living on that land. You tell them "I own this land, if you want to live you can either leave (flee as refugees), or trade me your right to freedom and security for the permission to use my land, with the following caveats". I say "plus ethics" because there was an ongoing debate throughout the feudal period about what obligations the landed class had to the working class, a debate that is taken less seriously today.
And for that matter, cronyism is consistent with pure capitalism; why shouldn't I, the holder of capital, be allowed to allocate that capital however I like, including to my cronies?
I apologize if this is all sort of obvious, or pedantic. And overall, I favor socio-political systems that are more capitalist than anything else. However, as I said in the tl;dr, I believe that the "I favour pure capitalism" mindset is harmful, if you actually try to use it to decide between policy alternatives.