Ken Lewis’ latest disclosure that as Bank of America CEO he hid key information from shareholders makes us want to look back at the bank’s deal to buy Merrill Lynch. But first, let’s look forward. What does the whole episode, which wiped out tens of billions of dollars in shareholder value, suggest about our financial system in 2012 and beyond?

The answer is clear: If a company is too big to fail, as Alan Greenspan pointed out, it is too big. Four years after the banking meltdown, policymakers still debate how and even whether to tighten government regulation of the financial sector. Of course we should, and we should start with one common-sense reform: Banks should not be allowed to gamble federally insured deposits on risky investments that ultimately gore taxpayers when they bomb.

As UNC Charlotte banking professor Tony Plath argued to the Observer editorial board Monday, banks can gamble all they want – but they and their shareholders should bear the risk, not the taxpayers. That can be forced by separating the federally insured deposits from other investments. Federally backed deposits should be placed strictly in safe, simple investments such as treasury bills. The part of a bank that invests in dicey assets should face the same market forces as other firms, up to and including going out of business. It’s a change that can and needs to be made globally, because doing it only in America would put U.S. banks at a considerable disadvantage.

Without that adjustment, banks will continue their “moral hazard” – engaging in high-risk, high-payoff behavior because the payoff goes to the bank while the risk is borne by others.

In Bank of America’s case, both shareholders and taxpayers endured the risk. The New York Times reported late Sunday that BofA shareholders were misled about Merrill Lynch’s financial health before they approved the $50 billion deal in December 2008. In sworn testimony filed in a shareholder lawsuit in federal district court in Manhattan, Lewis conceded that he knew the deal was projected to hurt BofA’s earnings more than the bank had told shareholders in proxy statements.

Voting without full knowledge, shareholders approved the deal, ultimately forcing a second $20 billion federal bailout and triggering a hammering of BofA stock from which it has yet to recover.

Some bank executives had urged full disclosure. Then-bank treasurer Jeffrey Brown apparently warned then-CFO Joe Price that failing to do so could be a criminal offense. General Counsel Tim Mayopoulos was fired a day after he questioned the lack of disclosure, court documents show.

So who is to blame? The law firm Wachtell, Lipton, Rosen & Katz, which advised against disclosure? Lewis and other BofA executives and board members, who accepted that advice? Or, perhaps most of all, then-Treasury Secretary Henry Paulson and other federal officials who bullied Lewis into doing the deal and keeping quiet about Merrill’s mounting losses?

All of the above were accomplices, bound together by a financial system that had spiraled wildly out of control, forcing Paulson, Lewis and others to take extraordinary measures that at a minimum pushed legal boundaries.

In July 2009, Paulson said: “Learning the lessons of the past is a necessary and important step as we forge our way forward.” Three years later, it’s no less true.