Coporations need to make a profit. I don't think this necessarily makes them evil. The ones that you mention make a profit at the expense of people's wellbeing and happiness, so their profit motive definitely puts them in conflict with the better good of people.
I would say corporations are generally amoral, not evil. This is why regulations for corporations are so important; they guarantee an appropriate way to make profit without say polluting all the drinking water in the country.
In the case of the prisons and medical insurance, the profit motive is directly in conflict with the actual positive desired outcome. In cases like this it's clear that the government should be the service provider, where the desire is to reduce costs, not see them increase for profit.
I guess for me, people can be evil, corporations aren't themselves capable of evil without people.
One thing I would like to see more of is individuals who perform evil acts inside companies be held personally liable. To many walk away without any consequences, see Texaco and Venezuela.
In terms of slave owners, the person is evil, and so was the system. I suppose you have a point that corporations can be evil in some cases, but I'd generally say that is because of a lack of regulations to restrain them, but this is obviously a gray area.
I would say corporations are generally amoral, not evil. This is why regulations for corporations are so important; they guarantee an appropriate way to make profit without say polluting all the drinking water in the country.
In the case of the prisons and medical insurance, the profit motive is directly in conflict with the actual positive desired outcome. In cases like this it's clear that the government should be the service provider, where the desire is to reduce costs, not see them increase for profit.