My understanding of it is that the crisis had a lot of proximate causes, and most of the people responsible were not acting in bad faith. The list includes:
Greenspan’s policies at the Fed. Low interest rates probably encouraged a number of bubbles, including the housing bubble. It made residential mortgages an attractive investment.
The growth of Asia. From 1993-2006, a number of Asian countries experienced wild growth, leading to what This American Life called “The Giant Pool of Money” which ultimately encouraged massive investment in residential mortgages.
Good old-fashioned ignorance. This includes both consumer ignorance among the people who took out loans they couldn’t afford, and ignorance among people who incorrectly thought that home prices were legitimately rising and would do so infinitely. It also includes ignorance on Wall Street about how CDOs worked, and how risky they were. Finally, this category also includes the failure of imagination to ask what would happen if a firm like AIG failed and prepare for that possibility.
Federal policies on housing. Federal policies subsidizing home ownership fed the housing bubble, and eventually (though somewhat late in the game) Fannie and Freddie got in on the act which made the situation worse.
Dishonest bankers. When mortgages became a hot item, many lenders changed their lending practices so they could issue more mortgages. Some had neutral intentions. But others committed fraud, or at least knew that the loans they were making were riskier than they let on to either the lendee or the company purchasing the loan. Similarly, there were people on Wall Street who turned a blind eye to the risks created by home asset CDOs, knowing that they would benefit short-term.
Conflicts of interest. Part of the reason home asset CDOs were not identified as the massive risk they were was because credit rating agencies had an interest in not assessing them properly.
A Wall Street culture too focused on the short-term. The rewards in the financial industry for most people employed there (who didn’t have a million dollars to place bets), did not go to people who would sit down and question the wisdom of home asset CDOs or current lending practices.
Bank leverage problems. Banks made bad decisions with respect to their leverage rations. A combination of general overleveraging caused by other factors and the rise of credit default swaps made financial institutions uniquely vulnerable to the popping of the housing bubble.
Lack of regulation. There should have been better regulation of everything from lending standards to transparency regarding CDOs, and a number of things in between. There wasn’t both because of deregulation and because of a failure to keep up with financial innovation. There was also a failure to enforce what regulations did exist, in part because the SEC is very much subject to regulatory capture. This includes the repeal of Glass-Stegall, which contributed to “Too Big to Fail.”
If I had to assign blame points, I would assign them in equal measure based on whether the agent acted with bad intentions and how much each agent was responsible. So, for example, China bears no blame for making a bunch of people rich. Greenspan bears a little blame for being so ideological. Dishonest lenders should get a lot of blame, even though they were only a small part of the responsibility.