It's not Wall Street. Wall Street only took advantage of the opportunity.
It's our government/congress that is at fault. Congress has been pushing the agenda of home ownership for the poor to the extreme for decades. In order to make it possible for "the poor" to own a home Congress relaxed the lending standards for home mortgages ... resulting in 'no documentation' loans aka 'liar loans' available to anyone.
The ACORN organization was instrumental in advising people how to obtain mortgages without any proof of income. Basically, people could get a mortgage just showing a driver's licence. The generated mortgages were then sold to the government's Fannie Mae and Freddie Mac.
Wall Street stepped in and repackaged the Fannie Mae and Freddie Mac mortgage paper as derivatives and sold them world wide. AIG insured much of the derivatives products.
When those people who should not have gotten mortgages in the first place began defaulting on their mortgage payments in droves the real estate market took a dive.
The derivatives market collapsed. The outstanding derivatives became nearly worthless with many of the underlying mortgages failing to perform. Nobody wanted to touch them.
AIG the insurer of the derivatives was unable to meet its obligations. Banking institutions here and abroad holding the derivatives had to take write-downs wiping out their equity.