I don't think of finance like I think of big tobacco though, and the criticisms I hear of finance tend to fall into one of two camps (i) lumping all of finance into one category; (ii) not really understanding what finance does. There are some areas of finance that are morally dubious (creating ever-more-arcane financial instruments that people don't understand). And I'm sympathetic in general to the idea that a lot of finance is rent-seeking, in which case you want regulation to cut down on that. But you don't have to go into dodgy areas of finance. Matt primarily takes advantage of arbitrage opportunities; it's hard to see how that is fucking over the world.
And even on the Big Tobacco front: if you could go in and substantially change their policies for the better (even if you couldn't make them any way close to perfect), I'd think of that as a really honorable thing to do. The same goes in finance. If you do find yourself in a really dodgy situation, you can always be a whistleblower, and potentially do a lot of good that way.
I like their estimate #1. For earning to give in finance to be net-negative then finance would have to be causing net harm approximately equal to all the deaths in the world.
What did you think of Brook Allen's response: http://qz.com/57807/dont-come-to-wall-street-for-the-money-e...
His point was that the actual making of money might actually cause more harm (fraud, etc) than the good your donations would do. I don't agree; objectively weighing the harm with the benefits is possible, as other comments have pointed out.