each system is as violent as the people running it. An ideal king is just the land owner collecting rents from the people on their land, and lending rights to people they think can make more wealth from it.
Capitalism is perfectly happy with slavery; it didn't end because it wasn't profitable.
There's still some rather violent things in today's capitalism like "give me all your money or die from cancer" which when the capitalists gave you cancer, isn't much different than the violent king forcing you to farm. You give them what they want or you die.
Actually, it ended because it could not compete with free labor. It was dying around 1800 because of that, and then the cotton gin revived it. It was dying again in the 1850s, and a big driver of the Civil War was the slave states trying to protect their economy from free labor competition.
I don't think it's a coincidence that social justice has tended to follow what works best economically.
Ask yourself why the USSR was unable to compete with the US economically, and could not even feed itself (Kansas in the 1970s was known as "the breadbasket of the Soviet Union". Why the Chinese could not compete with the US until they switched to free labor. Why N. Korea is starving and regular shipments of food from the west keeps things from getting much worse. The Middle East is not known for exporting factory goods.
The examples go on and on.
Where forced labor still exists there is something else propping it up or subsidizing it - like I mentioned with prison labor.
Claiming that labor in the USSR was slave labor is disingenuous and quite a stretch. By the same standard, I can claim that Western wage labor was also slave labor.
I'm also dubious of your Kansas claims (Google search comes up with nothing). Again, by the same standard, I can claim that the US "cannot even clothe itself" today. I'm sure reality is a lot more nuanced than that.
I can expand further on history of the USSR and long-term effects of WW2 on that country's population, which had massive waves of effect well into 1990s, but I'll stop here.
The war didn't just happen. It didn't start out as a war to end slavery, either (that happened later, with the Emancipation Proclamation).
As for prison labor, most of the cost of that is borne by the taxpayer, not the company using their labor. Keeping people prisoners in the US is very expensive.
Nothing you just said supports that slavery ended because of the "free market".
1) The Civil War was absolutely about slavery. Here's a professor at West Point talking about: https://youtu.be/pcy7qV-BGF4 (and PragerU is a very conservative source too).
2) The Emancipation Proclamation didn't end slavery. It ended it in states that were still rebelling. It took amendments to the Constitution to end slavery once and for all.
3) I agree that keeping prisoners is very expensive in the US and that most of that cost falls on the taxpayer. But it is very much still (essentially) slavery that the "free market" uses.
Capitalism is perfectly happy with slavery; it didn't end because it wasn't profitable.
There's still some rather violent things in today's capitalism like "give me all your money or die from cancer" which when the capitalists gave you cancer, isn't much different than the violent king forcing you to farm. You give them what they want or you die.