• Refusal by Clinton and Bush 43 administrations to regulate derivatives, CDOs, CDSs, and other new financial instruments
• Greenspan keeping interest rates very low, which made lending for mortgages cheap, which helped inflate the bubble
• Failure of the ratings agencies to properly identify risk
• Failure of bank VAR systems and other internal mechanisms to identify risk
• Mortgage originators, who had no skin in the game, and quickly sold off new mortgages with poor documentation
• Mortgage originators and banks who thought it'd be a good idea to increase the number of subprime and exotic loans
• Outright fraudulent mortgage originators and banks, who just wanted warm bodies to sign loans
• Mortgage risk evaluation software, which was using out-of-date and inaccurate risk models
• Investment banks, who also often had no skin in the game, and quickly sold off new mortgages with poor documentation
• Investment banks that did everything they could to make their offerings as obscure as possible, and sell them not just to financial actors who should have known better, but also to people who had no business buying financial instruments they didn't understand
• Aforementioned financial entities that bought total crap without looking into it enough
• Aforementioned clueless buyers who had no business buying MBSs and CDOs and derivatives
• Real estate speculators, who took advantage of and fed the bubble
• Much of the American public, who were buying properties they couldn't afford, with mortgages they should never have taken out in the first place
• Pretty much everyone, who did not realize that there was a huge bubble in real estate
Despite conservative screeching, Fannie & Freddie did very little. They were very late to the subprime market, and basically -- like Lehman -- were caught holding the bag.
The government is actually more responsible for inaction -- failure to oversee new markets, rein in banks, and pop the bubble early -- than for actions. But that's very typical in a bubble. No one is ever rewarded for causing a recession earlier, even if doing so would reduce its intensity.
And that's the truth. In a nutshell. A bit more complex than your narrative, but don't let that bother you.