Ending explained. Possibly.
At the start David loses the customers retirement money Corey tells him not to stress over it because 'everyone is screwed'. I took that to mean their company has lost a lot of money for a lot of clients.
Given the planning involved in the setup and the killer's filing cabinet full of other plans I figured he was a former client who lost everything and is now taking revenge on the employees of the company one by one.
When I consider this theory I don't find the movie that bad (although it was pretty dumb haha). On the other hand it was full of holes. The simple solution to the whole thing was to quickly sneak away while the killer was banging around the back of the ATM. Maybe the guy's next victims will be smarter.