I'm aware of democracy's tendency to go astray through demagogery, as I write about in the articles linked below. Corporations are hangovers from monarchies, though, due to their undemocratic distribution of internal power, and they have every incentive to get away with as much wrongdoing as they can in capitalism--which is a lot when they amass enough wealth and power to dictate much of what happens in the government and the judiciary (through lobbying, the revolving door, distortion of political campaigns, etc).
So asking which sector is more destructive, the public or the private, is futile because this is often based on a false dichotomy. The more laissez-faire the economy, the greater the private sector oligopolies' control over the democratic process. Protection of the environment is mainly a public sector concern, and it's a popular one. But to the extent that the change isn't happenning fast enough, that's due to private sector veto power over democratic governments.
The most destructive governments are probably the tyrannical ones, with centralized power, as you say. But that power structure has been preserved in large corporations which are hardly democratic. So by condemning dictatorships, you're implicitly condemning big businesses.
