Ask yourself where she got the gun? Remember what she was taught about the mark's mindset when the con is over? The gun had blanks and it was provided to her from the very beginning.

When the patient comes back at the end she was SUPPOSED to see him drive away in the red convertible and lead her to the gang splitting up her 80 thousand.

The patient was in on the con from the beginning.

Mantegna does not die in the end - the gun had blanks.

There - enough spoilers for you there? This is why people are giving it such high ratings. It's extremely original because of the hidden ending and how it cons MOST of the audience.