HOW DID BELLA DIE?

Molly Weasley killed her after Bellatrix was dueling Luna, Hermione, and Ginny together, and she shot a killing curse to Ginny but missed and aggravated by the death of Fred she doesn't want her ONLY daughter dead so she shot a curse that hit Bellatrix on the chest.

I can't wait July 2011 to watch that particular scene - Bella and Molly's duel scene. That would be the final end of Bella at Harry potter and the deathly hallows part 2, to be shown in cinemas next year - the last franchise of the Harry Potter movie series by Warner Bros.

"What will happen to your children when I've killed you?" taunted Bellatrix, as mad as her master, capering as Molly's curses danced around her. "When Mummy's gone the same way as Freddie?"
"You-will-never-touch-our-children-again!" screamed Mrs. Weasley.
Bellatrix laughed, the same exhilarated laugh her cousin Sirius had given as he toppled backward through the veil, and suddenly Harry knew what was going to happen before it did.
Molly's curse soared beneath Bellatrix's outstretched arm and hit her squarely in the chest, directly over her heart.
Bellatrix's gloating smile froze, her eyes seemed to bulge: For the tiniest space of time she knew what had happened, and then she toppled, and the watching crowd roared, and Voldemort screamed.
- Harry Potter and the Deathly Hallows, Chapter 36: The Flaw in the Plan, pages 735-737.

I actually rather hope that she DID use Avada Kedavra to kill Bellatrix... and I don't see any problem with it at all. Killing an enemy in self-defense, especially an enemy that is trying to kill you, is not the same as murder (as it seems we agree since most people are OK with the idea she definitely killed Bellatrix somehow). And the fact that AK is "illegal" has kind of flown out the window with the collapse of the Ministry of Magic.

It's only an "Unforgiveable Curse" because the Ministry decided it should be so (with good reason, certainly); but this is war, and it's simply a fact that you can't always hope to win by only stunning and disarming your opponents. Killing an enemy in self-defense is perfectly acceptable, and the method of doing so (Killing Curse vs. Muggle-weapon) is really irrelevant. There's nothing at all about Avada Kedavra that makes it any worse than any other method of killing an opponent, it all comes down to the reasons why it is used.

I DO have issues with using Crucio for any reason whatsoever (I don't think that full use can ever be justified under any circumstances), and I'm still debating the use of Imperio (since it involves the overriding of another person's free will), but there is no doubt in my mind when it comes to the Killing Curse: given sufficient reason, it is acceptable to use against an enemy in self-defense, and this situation more than qualifies.