Of all of the things to be pissed about in DC, this isn't one of them IMO. This isn't the revolving door between regulator and the regulated industry. This is just some high level guy in government moonlighting in a mostly unrelated industry to make some coin on the side. This should be no more offensive to most than a GS14 or GS15 technical staffer taking out a contract with a big corporation on the side to make some extra bucks.
One thing the people crucifying Alexander and his company seem to forget is that if he's actually parlaying his background at the NSA into making the banks better at security, then that's a net gain for the American people. Be pissed all you want about what he did in the past, but the fact is for all we know he's also advising his clients on how to become more "NSA-proof" on the down low. I would be very surprised if he a bank offered him a lot of lucre to make them harder for intelligence services to breach that he'd suddenly turn that down and go squealing to Fort Meade now that his paycheck comes from the private sector.