There are plausible explinations
I know this movie is sort of a stretch for most of us, but most of what occurred can be explained, as the premise is set in the movie. I think what most of you are forgetting, or disregarding, is that the badguy is planning his assaults to the finest of details. He's catching every angle, considering every contigency. When you take into consideration that he really went over every fine detail, it starts to make a bit more sense. Here's my take...
1. Why didn't they just all run in different directions? Because they each had enough fear of being the one to get caught, none of them wanted to take the chance. Remember, these guys weren't inner-city toughguys or US Marines. These were office workers. Not buffed out MMA atheletes.
2. Why didn't the two men leave the ATM and fight off the villian? This was an option conveyed by one of the characters. But the female demanded they not leave her alone, and the one guy that had a crush on her wanted to essentially do whatever she wanted.
3. Why didn't the security guard act quicker? This guy likely never had any odd things happen since he got the job. Security work is alot more boring and routine than you think. It was a new thing for him to come across and actualy incident, and he wasn't sure of what was going on. It could have been a prank. He took a realistic amount of time to assess the environment, and then he acted when he saw the body. That makes sense to me. And remember; he was a security guard, not a trained police officer.
4. What was the dog walker doing out with his dog so late? Where I live, I see people walking dogs at all hours of the night. Some people are night owls. Some people get off work late, and their regular activties extend passed midnight. I didn't find this to be a stretch.
5. Why no exterior cameras? Remember, the badguy recons his target areas very deligently. He certainly chose a vestibule that didn't have exterior cameras. Or, he could have removed said cameras, or disabled them.
6. How did the vestibule fill up with water? Again, the badguy did his homework. He likely inspected the facility to ensure it was near-watertight. It's not a stretch if you consider that the badguy planned this thing out to the tee.
7. Why did they park so far away? Now, this one I don't know. Maybe handicap parking?