The way the ending was filmed was a mistake! (spoilers)
it was not clear to a majority of people who watched this film what happened during the climax of the film !!(the gun backfires, killing her) -- i.e. the gun was rigged to backfire by George clooney's character.
And... the reason a vast majority of viewers did not understand this , is the failure by the director to make clear what happened. This could have been accomplished in a wide variety of ways (slow motion close up of the internal parts malfunction of the gun, for instance) , rather than going for "the understated approach" in a such a pivotal scene
p.s.
before you try to defend how clear the ending was, get a group of people to watch it and write down what they think happened, and see the responses you get.
p.p.s.
I propose they reshoot the scene and fix it like george lucas did to his movies