A few thoughts,

The previous post is only discussing income taxes. Everybody, rich and poor alike, pay other taxes too numerous to mention (gas,sales, property, etc.) Many of those are "flat taxes".

Simply being rich does not make some one "evil or mean" it's how they became rich and how they comport themselves after earning their wealth. Rich and poor alike can be evil and mean, but with more money, you make a bigger splash.

And, finally my reasoning for agreeing with a progressive income tax. I do disagree with excessive upper rates (I believe some european countries have rates that max out at 90%, but I don't know for a fact). As much as conservatives like to go on about the perceived bias against the wealthy, our country is tilted in favor of those with money. The more money a person has the smoother society is for them, and the easier it is to earn more and save more. A couple of examples, the legal system favors  those who can afford to buy the best lawyers (if O.J. had to use a public defender he'd be contemplating a needle right now); Buying items on credit, more money lower interest rates, or pay cash and save even more, the list goes on and on. Whether someone earned their millions or inherited it, unless they totally screw things up they've got it much easier once they have money.

Of course progressive taxes have a socialist tinge, our country is not now, nor ever has been, based on pure captialism, democracy, federalism, or free market theory. I don't think we would want that. We are on a continuum, the goal is to not stray to far either way, keeping the majority of people content, with some opportunity, and avoid large groups of the population disaffected and with nothing to live for. If that group gets too big you end up with riots or revolution.