In the movie Harry Potter and the Deathly Hallows part 2, Molly Weasley used Duro to turn Bellatrix into stone. Then she used Reducto to blast Bellatrix into a thousand pieces.
In the book however, it didn't say, but was most likely not Avada Kedavra (the killing curse), as she would have been sent to Azkaban for it.
to the guy above- There was a war going on. Why would she have been sent to Azkaban for protecting her country? Now if that war hadn't been going on and she had done it to an enemy when there wasn't anything true going on they might do it to avoid panics but that was a war. People do horrible things during wars to protect their people. She wouldn't have been sent to Azkaban for defending the students against people who openly said to everyone in the world that they're going to kill all mortals and all except purebloods. there was no negociations, no smaller fights, no strick ops that would no one would respond to. There was absolute war I thought in the movie that she had used the killing curse so I wouldn't have been surprised if she had in the books.