This really is completely useless without being able to read the study, though. If you put greed (ie, interest in obtaining money) as a solely unethical want, it's obvious that the people who have gotten money are greedy by that definition.
But if they had flipped it around, it can easily be seen the other way too. Rich people get rich by doing something beneficial for society. Whether it's betting your money that a company is going to be successful, or actually making the company successful. They do something that makes the world better, and get a personal payoff for that. The rich people are often the ones who made the world a better place on a large scale.
Honestly, the "the rich are evil" philosophy just seems more like a way of justifying that other people gained more money than themselves. "They were only more successful than me because they exploited other people!".
The way I see it, the result is simple. The majority of the rich people did one hell of a lot more good to the world than the majority of the people with less money. And, they got money back for doing that. If you call that greed, that's fine, I just think it's perfectly fair (in the majority of cases).