As a former member of the financial world... what can I say....
This movie does an amazing job of breaking down every reason I left that industry. Shadiness, greed, actuarialization and ruthlessness combined with a corporate/political compartmentalization that makes it impossible for any one group, let alone a single person, to understand the full problem... and why it impacted the entire world... and really... continues to do so almost 12 years later (at the time of this posting).
It only tells part of the picture though. The scope of the problem goes way beyond just the short sellers. You would have to watch a few different films and documentaries to get even half the picture.
As far the movie's depiction of how things work behind the scenes... I think they were pretty spot on... taking a few liberties here and there... but mostly this is an accurate depiction of how the shady deals are done with nothing but bonuses and profits in mind... even to the point where the perpetrators have convinced themselves that what they are doing is right... because... profits.
Profits = Higher Stock Prices
Higher Stock Prices = Happy Shareholders
Happy shareholders = A Job Well Done.
They really don't think about it beyond that when justifying their actions, outrageous bonuses, powerful lobbies, and crooked dealings that pillage the world's wealth.
This movie shows a little bit of each part of that chain of thoughts and actions that lead to the housing collapse... which in turn caused Too Big To Fail (watch on HBO, 5-stars)... which lead to the Occupy/Tea Party Movement... which then lead to Donald Trump.
If you want to learn more... and go back further to understand what caused these events... look into the deregulation of the banks and financial firms from Reagan to Bush Jr.