It was pretty contrived…establishing at the outset that the villain is a dumbass who habitually leaves his car unlocked.

Kristen shot him. You know it was her cuz Gaines shakes his head the way he always did when he'd see her. And the shotgun was situated in the same part of his face that Gaines gestured in court when Kristen was testifying against him.
It was her. It was revenge for Gaines killing the man she loved.

I think it might be a bit tasteless, but it was consistent with the storyline, since Patric's character told her had someone kill him, he would like her to do him the favor of kill that guy. That last scene is actually implicit of something exposed before. Makes sense now.
now this is acting: http://www.imdb.com/media/rm2458172160/tt1528718

it's not ridiculous. what would you rather? gaines go to jail and kristen just quits the force? that's totally boring.
Not as boring as the Neanderthal eye-for-an-eye justice this current ending espouses.

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The explanations are pretty convincing. I wish that that wasn't the case.
Horrible horrible ending. Totally tasteless, totally ridiculous, leaves a bad taste in my mouth.

everyone explain it well to u. kristen killed him.
when she saw him run his fingers along his face like
gunman did her in trailer and her partner was killed
she knew it was gaines so she freed him by saying she
and partner planted evidence against him.then at end
when hes driving down rainy road and hears someone
in his car and stops to look back .BANG A DOUBLE
BARREL SHOTGUN GOES OFF IN HIS FACE. justice.

kristen kills gaines same way he killed her
partner. not hard to figure out.a double barrel
blast in the face.justice.thats why she told them
they had planted evidence on him so he could be
free and she could blow him away.still was a good
story only thing once a cop takes drugs during
undercover all evidence is useless.she knew her
only way to get him was kill him.

If you remember the scene in bed Jim tells Kristen that if someone kills a partner that he wouldn't leave it up to the courts and so on.

And that was the ultimate spoiler

This is my take on the movie. If your talking about Gregory driving the Mercedes, there is someone in the back seat, however he did NOT kill that person. They killed him. The person in the back of the car is Jennifer Jason Leigh. If you remember when Jason Patrick was killed? Well Jennifer had to go to court to testify against Gains who was Allman. If you watch the court room seen at the end where she looks at Gains, he is taking his finger and running it down his nose and face just like the gun was done on her face in the trailer when Patrick got killed. She then knew he was the killer and set him free by ratting on the cops so she could kill him in the end. This is my take on what happened, I have watched this so many times and get the same thing every time. Hope this helps you out.
psywizard880

She also knew she would be going to prison? (For lying to the grand jury) Prison or probation she was in trouble.

No, Kristen shoots him