The far more significant cause of the Great Recession was outright fraud. Watch https://www.youtube.com/watch?v=ClfBxWPkBKU and other videos featuring William Black (a former bank regulator from the S&L crisis days).
Wikipedia discusses a bunch of factors related to the Great Recession, correctly mentioning that different economists place different weight on them: https://en.wikipedia.org/wiki/Causes_of_the_Great_Recession
Here's a video by Tyler Cowen that discussions the Great Recession in terms of four DIFFERENT macroeconomic models: http://www.learnliberty.org/videos/explaining-the-great-rece...
Lastly, I'll say that even talking about causation generally is difficult. If someone buys a gun and shoots me, what was the cause of me getting shot? If guns had been outlawed, I wouldn't have been shot. Is it Congress's fault? If the shooter hadn't pulled the trigger, I wouldn't have been shot. Is it the shooter's fault? If I hadn't failed to jump out of the way, I wouldn't have been shot. Is it my fault? Defining causality when there are many serially dependent steps is not easy. So when someone asks, why did the Great Recession happen, it's hard to even know what the question means. Feynman has a famous interview segment where he talks about the difficulty of answering 'why' questions: https://www.youtube.com/watch?v=Dp4dpeJVDxs
Murder is also illegal. In your hypothetical scenario, you're assuming the shooter was willing to break the law to murder you, but would not have been willing to use an unlicensed gun to do so?